drm/i915: Nuke pointless variable

No point in assigning the function return value to a local
variable if we're just going to use it the one time.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200630215601.28557-13-ville.syrjala@linux.intel.com
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>