accel: Add .mmap to DRM_ACCEL_FOPS

In reviewing the ivpu driver, DEFINE_DRM_ACCEL_FOPS could have been used
if DRM_ACCEL_FOPS defined .mmap to be drm_gem_mmap.  Lets add that since
accel drivers are a variant of drm drivers, modern drm drivers are
expected to use GEM, and mmap() is a common operation that is expected
to be heavily used in accel drivers thus the common accel driver should
be able to just use DEFINE_DRM_ACCEL_FOPS() for convenience.

Signed-off-by: Jeffrey Hugo <quic_jhugo@quicinc.com>
Reviewed-by: Oded Gabbay <ogabbay@kernel.org>
Signed-off-by: Oded Gabbay <ogabbay@kernel.org>