drm/i915: Fill out the FWx watermark register defines

Add defines for all the watermark registers on modernish gmch platforms.

VLV has increased the number of bits available for certain watermaks so
expand the masks appropriately. Also vlv and chv have added some extra
FW registers.

Not sure what happened on chv because a new register called FW9 is now
at the offset where FW7 was on vlv, while FW7 and FW8 (another new
register) have been moved off somewhere else. Oh well, well just need
two defines for FW7 then.

v2: Fix DSPHOWM1 offset (Paulo)

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>