drm/vkms: fbdev emulation support

drm/vkms: fbdev emulation support

Hooray for generic fbdev support, making this a oneliner. We just
needed to fix preferred_depth fixed and the vmap support added first.

This is useful for testing fbdev testcases in igt. Right now we only
have a simple one in the fbdev testcases, which passes both info and
mmap subtests.

v2: Augment commit message as suggested by Melissa.

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Melissa Wen <melissa.srw@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Cc: Melissa Wen <melissa.srw@gmail.com>
Cc: Haneen Mohammed <hamohammed.sa@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20201009232156.3916879-3-daniel.vetter@ffwll.ch