drm/vc4: Use drm_gem_cma_vmap() directly

Validated shaders cannot be exported. There's no need for testing this in
the BO's vmap implementation. Call drm_gem_cma_vmap() directly instead.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210108140808.25775-2-tzimmermann@suse.de