drm/i915: Always use cpp==4 for FW_BLC_SELF on 915GM/945GM

drm/i915: Always use cpp==4 for FW_BLC_SELF on 915GM/945GM

Bspec says:
"FW_BLC_SELF
 ...
 Programming Note [DevALV] and [DevCST]: When calculating watermark
 values for 15/16bpp, assume 32bpp for purposes of calculation using
 the high priority bandwidth analysis spreadsheet."

Let's do that.

Perhaps this might even help with the problem that resulted in
commit 2ab1bc9df01d ("drm/i915: Disable self-refresh for untiled fbs on i915gm")

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1469804222-12650-1-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>