drm: update support for drm pci buffers

The DRM needs to change the drm_pci interface for FreeBSD compatiblity,
this patch introduces the drm_dma_handle_t and uses it in the Linux code.

From: Tonnerre Lombard, Eric Anholt, and Sergey Vlasov
Signed-off-by: David Airlie <airlied@linux.ie>