drm/gem: Provide drm_gem_fb_{vmap,vunmap}()

Move framebuffer vmap code from shadow-buffered plane state into the new
interfaces drm_gem_fb_vmap() and drm_gem_fb_vunmap(). These functions
provide mappings of a framebuffer's BOs into kernel address space. No
functional changes.

v4:
	* remove duplicated blank line
v2:
	* using [static N] for array parameters enables compile-time checks
	* include <drm/drm_fourcc.h> for DRM_FORMAT_MAX_PLANES (kernel
	  test robot)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210730183511.20080-3-tzimmermann@suse.de