drm/i915: Clean up intel_find_initial_plane_obj() a bit

Sort out the mess with the local variables in
intel_find_initial_plane_obj(). Get rid of all aliasing pointers
and use standard naming/types.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210609085632.22026-4-ville.syrjala@linux.intel.com


Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>