drm/gem: Move mapping of imported dma-bufs to drm_gem_mmap_obj()

Drivers that use drm_gem_mmap() and drm_gem_mmap_obj() helpers don't
handle imported dma-bufs properly, which results in mapping of something
else than the imported dma-buf. For example, on NVIDIA Tegra we get a hard
lockup when userspace writes to the memory mapping of a dma-buf that was
imported into Tegra's DRM GEM.

To fix this bug, move mapping of imported dma-bufs to drm_gem_mmap_obj().
Now mmaping of imported dma-bufs works properly for all DRM drivers.

Cc: stable@vger.kernel.org
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>