drm: arcpgu: Fix mmap() callback

Now that ARC properly supports DMA mmap() we can use the standard
CMA helper to map dumb buffers. This makes ARC PGU works with
standard DRM consumer applications like, for example, mpv/mplayer
via DRM. While at it, use the DEFINE_DRM_GEM_CMA_FOPS() helper.

This fixes the use of dumb buffers.

Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Fixes: 0c4250e7b15e ("drm: Add support of ARC PGU display controller")
Cc: Carlos Palminha <palminha@synopsys.com>
Cc: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Dave Airlie <airlied@gmail.com>
Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>