drm/i915: Use FW_WM() macro for older gmch platforms too

Use the FW_WM() macro from the VLV wm code to polish up the wm
code for older gmch platforms.

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>