drm: Convert drm_legacy_ctxbitmap_init to void return type

It can't fail really.

Also remove the redundant kms check Peter added.

Cc: Peter Antoine <peter.antoine@intel.com>
Reviewed-by: Peter Antoine <peter.antoine@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

diff --cc drivers/gpu/drm/drm_context.c
Simple merge

diff --cc drivers/gpu/drm/drm_drv.c
Simple merge

diff --cc drivers/gpu/drm/drm_legacy.h
Simple merge