drm/msm: fix a6xx_gmu_clear_oob

The cleanup patch broke a6xx_gmu_clear_oob, fix it by adding the missing
bitshift operation.

Fixes: 555c50a4 ("drm/msm: Clean up GMU OOB set/clear handling")
Signed-off-by: Jonathan Marek <jonathan@marek.ca>
Signed-off-by: Eric Anholt <eric@anholt.net>