drm/vram: Add helpers to validate a display mode's memory requirements

drm/vram: Add helpers to validate a display mode's memory requirements

Devices with low amount of dedicated video memory may not be able
to use all possible display modes, as the framebuffers may not fit
into VRAM. The new helper function drm_vram_helper_mode_valid()
implements a simple test to sort out all display modes that can
not be used in any case. Drivers should call this function from
struct drm_mode_config_funcs.mode_valid.

The functionality was originally implemented by the ast driver, which
is being converted as well.

v2:
	* WARN_ON if VRAM memory manager has not been initialized
	* documentation fixes
	* unexported drm_vram_helper_mode_valid_internal()

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200203155258.9346-2-tzimmermann@suse.de