drm/i915/bxt: Fix sanity check for BIOS RC6 setup

drm/i915/bxt: Fix sanity check for BIOS RC6 setup

BXT BIOS has two options related to GPU power management: "RC6(Render
Standby)" and "GT PM Support". The assumption so far was that disabling
either of these options would leave RC6 uninitialized. According to my
tests this isn't so: for a proper RC6 setup we only need the "GT PM
Support" option to be enabled while the "RC6" option only controls
whether RC6 is left enabled or not by BIOS. OTOH we were missing a few
checks to ensure a proper RC6 setup. Add these now and don't fail the
sanity check if RC6 is disabled. This fixes a problem where RC6 remains
disabled after reloading the driver, since we explicitly disable RC6
during unloading.

v2:
- Print a debug message about the BIOS enabled RC state. (Sagar)

CC: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1467216835-1086-2-git-send-email-imre.deak@intel.com