drm/i915/fbc: Clean up all register defines

The FBC register defines are a mess:
- namespace changes between DPFC_, FBC_, and some platform
  specific prefix at a whim
- ilk+ reuses most g4x bits but still has some separate bit
  defines elsewhere
- it's not clear from the defines that the bit defines are
  shared

So let's clean it up:
- both g4x and ilk register share the same defines now
- only defines which conflict have a _PLATFORM suffix, everyone
  else just gets comments to indicate which platforms do what
- namespace is consistent DPFC_ now
- SNB system agent fence registers also get a consistent namespace
- REG_BIT() & co. for everything

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211104144520.22605-13-ville.syrjala@linux.intel.com
Acked-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>