845/865 support different cursor sizes as well, albeit a bit differently
than later platforms. Add the necessary code to make them work.

Untested due to lack of hardware.

v2: Warn but accept invalid stride (Chris)
    Rewrite the cursor size checks for other platforms (Chris)
v3: More polish and magic to the cursor size checks (Chris)
v4: Moar polish and a comment (Chris)
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>