drm/cma-helper: Implement mmap as GEM CMA object functions

The new GEM object function drm_gem_cma_mmap() sets the VMA flags
and offset as in the old implementation and immediately maps in the
buffer's memory pages.

Changing CMA helpers to use the GEM object function allows for the
removal of the special implementations for mmap and gem_prime_mmap
callbacks. The regular functions drm_gem_mmap() and drm_gem_prime_mmap()
are now used.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201123115646.11004-3-tzimmermann@suse.de