drm: Add drm_plane_mask()

drm: Add drm_plane_mask()

Add drm_plane_mask() which returns the 1<<index for the plane.
We already have an identical drm_crtc_mask() for crtcs.

Mostly performed with coccinelle:
@@
@@
- (1<<drm_plane_index(
+ drm_plane_mask(
  ...)
-  )

@@
@@
- 1<<drm_plane_index(
+ drm_plane_mask(
  ...)

@@
@@
- BIT(drm_plane_index(
+ drm_plane_mask(
  ...)
- )

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180626194716.12522-1-ville.syrjala@linux.intel.com
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>