drm/i915: Clean up PIPEMISC register defines

Use REG_BIT() & co. for PIPEMISC* bits, and while at it
fill in the missing dithering bits since we already had some
of them defined.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211112193813.8224-3-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>