drm/i915: Rename variables in intel_primary_plane_create()

Let's try to stick a common naming pattern in all the plane init funcs.

v2: Rebase due to color_encoding/range props

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181005125817.22576-12-ville.syrjala@linux.intel.com