drm/tilcdc: fix kernel panic on suspend when no hdmi monitor connected

drm/tilcdc: fix kernel panic on suspend when no hdmi monitor connected

On BeagleBone Black if no HDMI monitor is connected and suspend
is requested a kernel panic will result:

root@am335x-evm:~# echo mem > /sys/power/state
[ 65.548710] PM: Syncing filesystems ... done.
[ 65.631311] Freezing user space processes ... (elapsed 0.006 seconds) done.
[ 65.648619] Freezing remaining freezable tasks ... (elapsed 0.005 seconds) done.
[ 65.833500] Unhandled fault: external abort on non-linefetch (0x1028) at 0xfa30e004
[ 65.841692] Internal error: : 1028 [#1] SMP ARM
  <snip>
[ 66.105287] [<c03765f0>] (platform_pm_suspend) from [<c037b6d4>] (dpm_run_callback+0x34/0x70)
[ 66.114370] [<c037b6d4>] (dpm_run_callback) from [<c037ba84>] (__device_suspend+0x10c/0x2f4)
[ 66.123357] [<c037ba84>] (__device_suspend) from [<c037d004>] (dpm_suspend+0x58/0x218)
[ 66.131796] [<c037d004>] (dpm_suspend) from [<c008d948>] (suspend_devices_and_enter+0x9c/0x3c0)
[ 66.141055] [<c008d948>] (suspend_devices_and_enter) from [<c008de7c>] (pm_suspend+0x210/0x24c)
[ 66.150312] [<c008de7c>] (pm_suspend) from [<c008cabc>] (state_store+0x68/0xb8)
[ 66.158103] [<c008cabc>] (state_store) from [<c02e9654>] (kobj_attr_store+0x14/0x20)
[ 66.166355] [<c02e9654>] (kobj_attr_store) from [<c0185c70>] (sysfs_kf_write+0x4c/0x50)
[ 66.174883] [<c0185c70>] (sysfs_kf_write) from [<c018926c>] (kernfs_fop_write+0xb4/0x150)
[ 66.183598] [<c018926c>] (kernfs_fop_write) from [<c0122638>] (vfs_write+0xa8/0x180)
[ 66.191846] [<c0122638>] (vfs_write) from [<c01229f8>] (SyS_write+0x40/0x8c)
[ 66.199365] [<c01229f8>] (SyS_write) from [<c000e580>] (ret_fast_syscall+0x0/0x48)
[ 66.207426] Code: e595c210 e5932000 e59cc000 e08c2002 (e592c000)

This is because the lcdc module is not enabled when no monitor is detected
to save power.  However the suspend handler just blindly tries to save the
lcdc state by copying out the pertinent registers. However module is off
so no good things happen when you try and access it.

This patch only saves off the registers if the module is enabled, and
then only restores the registers on resume if they were saved off during
suspend.

Signed-off-by: Darren Etheridge <detheridge@ti.com>
Tested-by: Dave Gerlach <d-gerlach@ti.com>
Acked-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Jyri Sarha <jsarha@ti.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>