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>