drm/vc4: Initialize vc4_drm_driver with CMA helper defaults

The function vc4_prime_import_sg_table() is an otherwise empty wrapper
around CMA's drm_gem_cma_prime_import_sg_table(). Removing it in favor
of the latter allows to initialize vc4_drm_driver with CMA's initializer
macro.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210114084949.29014-1-tzimmermann@suse.de