drm/vkms: Enable/Disable cursor support with module option

Cursor support is not complete yet. Add module option 'enable_cursor'
to enable/disable cursor support which is used for testing currently.

Signed-off-by: Haneen Mohammed <hamohammed.sa@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/b47f44f518d3c9858f1469193f1136e0c490060b.1536210181.git.hamohammed.sa@gmail.com