drm/vkms: Add mode_config initialization

Initialize minimum and maximum width and height of the frame buffers
with default values.

Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Reviewed-by: Haneen Mohammed <hamohammed.sa@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/75c55df671f24b037f9172700b479f4bb2fa7c92.1526514457.git.rodrigosiqueiramelo@gmail.com