drm/vkms: Add support for writeback module

Add enable_writeback feature to vkms_config as a module.

Signed-off-by: Sumera Priyadarsini <sylphrenadin@gmail.com>
Reviewed-by: Melissa Wen <melissa.srw@gmail.com>
Signed-off-by: Melissa Wen <melissa.srw@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/15802da4f1cdfed2b728c3d35731732f161dd073.1610391685.git.sylphrenadin@gmail.com