drm/i915: Fix cursor 'cpp' in watermark calculatins for old platforms

drm/i915: Fix cursor 'cpp' in watermark calculatins for old platforms

The watermark code for the old platforms (g4x and older) uses the
primary plane cpp when computing cursor watermarks. To keep the fix
simple let's just hardcode cpp=4 for the cursor on those platforms
since that's all we support.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170421181432.15216-8-ville.syrjala@linux.intel.com
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>