drm/i915: Fix the g4x watermark TLB miss workaround

The g4x watermark TLB miss workaround requires that we bump up the
watermark by the difference between 8 full lines and the FIFO size.
Unfortunately the way we compute it at the moment ignores the size
of the pixels. The code also used the primary plane width as the
cursor width when computing the TLB miss w/a for the cursor.
Let's fix both problems.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170421181432.15216-9-ville.syrjala@linux.intel.com


Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>