drm: Wrap the check for atomic_commit implementation

drm: Wrap the check for atomic_commit implementation

This check is useful for drivers that do not have DRIVER_ATOMIC set but
have atomic modesetting internally implemented. Wrap the check into a
function since this is used in many places and as a bonus, the function
name helps to document what the check is for.

v2:
Change return type to bool (Ville)
Move the function drm_atomic.h (Daniel)
Fixed comment marker for documentation

Suggested-by: Daniel Vetter <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Ben Skeggs <[email protected]>
Signed-off-by: Dhinakaran Pandiyan <[email protected]>
[danvet: Move back to drmP.h because include hell.]
Signed-off-by: Daniel Vetter <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]