i965: Silence unused parameter warnings

Reduces my build from 7119 warnings to 7005 warnings by silencing 114
instances of

In file included from ../../SOURCE/master/src/mesa/drivers/dri/i965/brw_context.h:46:0,
                 from ../../SOURCE/master/src/mesa/drivers/dri/i965/intel_pixel_read.c:38:
../../SOURCE/master/src/mesa/drivers/dri/i965/brw_bufmgr.h: In function ‘brw_bo_unmap’:
../../SOURCE/master/src/mesa/drivers/dri/i965/brw_bufmgr.h:258:47: warning: unused parameter ‘bo’ [-Wunused-parameter]
 static inline int brw_bo_unmap(struct brw_bo *bo) { return 0; }
                                               ^~

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>

src/mesa/drivers/dri/i965/brw_bufmgr.h

@@ -252,7 +252,7 @@ MUST_CHECK void *brw_bo_map(struct brw_context *brw, struct brw_bo *bo, unsigned
  * Reduces the refcount on the userspace mapping of the buffer
  * object.
  */
-static inline int brw_bo_unmap(struct brw_bo *bo) { return 0; }
+static inline int brw_bo_unmap(UNUSED struct brw_bo *bo) { return 0; }
 
 /** Write data into an object. */
 int brw_bo_subdata(struct brw_bo *bo, uint64_t offset,