drm/i915: Add polish to VLV WM shift+mask operations

Wrap the FW register value shift+mask operations into a macro to hide
the ugliness a bit. Also might avoid bugs due to typos.

Also rename all the primary/sprite plane low order bit masks to have the
_VLV suffix, so that we can use the FW_WM_VLV() macro instead of the
FW_WM() macro for them in a consistent manner. Cursor and all the high
order bits are left to use the FW_WM() macro as there's no real
confusion with them.

Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>