drm/i915: Use u8 consistently for active_planes bitmask

Be consistent in that active_planes bitmask fits in a u8.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210514125751.17075-4-ville.syrjala@linux.intel.com


Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>