drm/i915: Generalize cursor size checks a bit

drm/i915: Generalize cursor size checks a bit

We have the maximum cursor dimensions stored in the mode_config, so
let's just consult that information instead of hardcoding the same
information in multiple places.

We still need to keep some per-platform checks as the limitations are
quite diverse.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170327185546.2977-10-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>