drm: add flag for mapping PCI DMA buffers read-only.

Add DRM_PCI_BUFFER_RO flag for mapping PCI DMA buffer read-only. An additional
flag is needed, since PCI DMA buffers do not have an associated map.

Signed-off-by: Dave Airlie <airlied@linux.ie>
diff --git a/drivers/char/drm/drmP.h b/drivers/char/drm/drmP.h
index 2f18329..0bbb04f 100644
--- a/drivers/char/drm/drmP.h
+++ b/drivers/char/drm/drmP.h
@@ -431,7 +431,8 @@
 	enum {
 		_DRM_DMA_USE_AGP = 0x01,
 		_DRM_DMA_USE_SG = 0x02,
-		_DRM_DMA_USE_FB = 0x04
+		_DRM_DMA_USE_FB = 0x04,
+		_DRM_DMA_USE_PCI_RO = 0x08
 	} flags;
 
 } drm_device_dma_t;