drm/i915: Always calculate 8xx WM values based on a 32-bpp framebuffer

The specs for gen2 say that the watermark values "should always be set
assuming a 32bpp display mode, even though the display mode may be 15 or
16 bpp."

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>