drm/ast: Check video-mode requirements against VRAM size

Each video mode's primary plane requires a minimum amount of video
memory. For double buffering, this is at most half the available
VRAM. Check this constraint.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191107083404.6852-3-tzimmermann@suse.de