drm/aperture: Add infrastructure for aperture ownership

Platform devices might operate on firmware framebuffers, such as VESA
or EFI. Before a native driver for the graphics hardware can take over
the device, it has to remove any platform driver that operates on the
firmware framebuffer. Aperture helpers provide the infrastructure for
native drivers to remove the generic ones.

For now, this only concerns generic fbdev drivers. Code for removing
these is provided by drm_fb_helper_remove_conflicting_framebuffers() et
al. Simply wrap these functions for now. At a later point, code can be
added for generic DRM drivers to acquire firmware framebuffers.

v2:
	* fix docs for drm_aperture_remove_framebuffers()

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210412131043.5787-2-tzimmermann@suse.de