drm/vc4: Move mmap implementation into GEM object function

Moving vc4's mmap code from vc4_mmap() into a GEM object function
allows for the use drm_gem_mmap() and drm_gem_prime_mmap(). The content
of vc4_drm_fpos can then be generated by DEFINE_DRM_GEM_FOPS().

The actual mmap implementation is just a check if the BO is a validated
shader plus the default CMA mmap code.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210108140808.25775-4-tzimmermann@suse.de