drm/i915: Clean up PIPESRC defines

drm/i915: Clean up PIPESRC defines

Use REG_GENMASK() & co. when dealing with PIPESRC.

Note that i9xx_get_initial_plane_config() will now use the
full 16 bit mask whereas previously it used 12 bits only.
But intel_get_pipe_src_size() already used the full 16 bits
on all platforms anyway, so at least we're consistent now.
The high bits beyond the max supported pipe source size
should not be set in any case so this seems fine.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211112193813.8224-7-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>