drm: Add local 'plane' variable for primary/cursor planes

drm: Add local 'plane' variable for primary/cursor planes

Make the code a bit more readable by storing the plane pointer in a
local variable rather than having to do crtc->{primary,cursor} all the
time.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180322152313.6561-6-ville.syrjala@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>