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/drm.h b/drivers/char/drm/drm.h
index 3f28a15..8db9041 100644
--- a/drivers/char/drm/drm.h
+++ b/drivers/char/drm/drm.h
@@ -356,7 +356,8 @@
 		_DRM_PAGE_ALIGN = 0x01,	/**< Align on page boundaries for DMA */
 		_DRM_AGP_BUFFER = 0x02,	/**< Buffer is in AGP space */
 		_DRM_SG_BUFFER = 0x04,	/**< Scatter/gather memory buffer */
-		_DRM_FB_BUFFER = 0x08	/**< Buffer is in frame buffer */
+		_DRM_FB_BUFFER = 0x08,	/**< Buffer is in frame buffer */
+		_DRM_PCI_BUFFER_RO = 0x10 /**< Map PCI DMA buffer read-only */
 	} flags;
 	unsigned long agp_start; /**<
 				  * Start address of where the AGP buffers are