i915g: add dma-buf support to i915_drm_buffer_get_handle

i915g: add dma-buf support to i915_drm_buffer_get_handle

The implementation of i915_drm_buffer_get_handle now handles
DRM_API_HANDLE_TYPE_FD in the same way that intel_winsys_import_handle
does, by calling drm_intel_bo_gem_create_from_prime.

Tested by successfully running Chrome's ozone_demo [1] with the
ozone-gbm backend on an Intel Pineview M machine. Without this change
it fails while trying to create a DMA-BUF.

[1] https://chromium.googlesource.com/chromium/src.git/+/master/ui/ozone/demo/ozone_demo.cc

Signed-off-by: Nicholas Bishop <nbishop@neverware.com>
[Emil Velikov: Fix coding style]
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>