drm/i915: Drop some more CHV pre-production workarounds

Drop WaDisablePwrmtrEvent:chv as it's no longer needed.

Also remove the WaSetMaskForGfxBusyness:chv note, but we still
leave the GEN6_RP_MEDIA_IS_GFX bit enabled as that's still the
recommended setting.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Arun Siluvery <arun.siluvery@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>