drm/msm/adreno/a6xx_gpu: Staticise local function 'a6xx_idle'

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/msm/adreno/a6xx_gpu.c:33:6: warning: no previous prototype for ‘a6xx_idle’ [-Wmissing-prototypes]

Cc: Rob Clark <robdclark@gmail.com>
Cc: Sean Paul <sean@poorly.run>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: linux-arm-msm@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Cc: freedreno@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>

drivers/gpu/drm/msm/adreno/a6xx_gpu.c

@@ -30,7 +30,7 @@ static inline bool _a6xx_check_idle(struct msm_gpu *gpu)
 		A6XX_RBBM_INT_0_MASK_RBBM_HANG_DETECT);
 }
 
-bool a6xx_idle(struct msm_gpu *gpu, struct msm_ringbuffer *ring)
+static bool a6xx_idle(struct msm_gpu *gpu, struct msm_ringbuffer *ring)
 {
 	/* wait for CP to drain ringbuffer: */
 	if (!adreno_idle(gpu, ring))