drm: Prefix info printk about registering panic notifier with 'drm'

drm: Prefix info printk about registering panic notifier with 'drm'

Recently I've studied my system dmesg and seen this:

  <lots of stuff before>
1 [    0.478416] ACPI: Battery Slot [C1B4] (battery present)
2 [    0.478648] ACPI: Battery Slot [C1B3] (battery absent)
3 [    0.906678] [drm] initialized overlay support
4 [    1.762304] Console: switching to colour frame buffer device 128x48
5 [    1.765211] fb0: inteldrmfb frame buffer device
6 [    1.765242] registered panic notifier
7 [    1.765272] [drm] Initialized i915 1.6.0 20080730 for 0000:00:02.0 on minor 0
8 [    1.765372] Serial: 8250/16550 driver, 4 ports, IRQ sharing enabled
  <lots of stuff after>

and it was not evident who registered that panic notifier on line 6.

I'd bought it as some low-level stuff needed by kernel itself, but the
time was inappropriate -- too late for such things.

So I had to study sources to see it was drm who was registering
switch-to-fb on panic.

Let's avoid possible confusion and mark this message as going from drm
subsystem.

(I'm a bit unsure whether to use '[drm]:' or 'drm:' -- the rest of the
 kernel just uses 'topic:', and even in drm_fb_helper.c we use 'fb%d:'
 without [] brackets. Either way is ok with me.)

Signed-off-by: Kirill Smelkov <kirr@landau.phys.spbu.ru>
Signed-off-by: Dave Airlie <airlied@redhat.com>