drm/i915: Eliminate redundancy in intel_primary_plane_create()

Lots of redundant assignments inside intel_primary_plane_create().
Get rid of them.

v2: Rebase due to fp16 landing

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191031165652.10868-8-ville.syrjala@linux.intel.com