drm/amdgpu: fixes a JPEG get write/read pointer bug

Need parentheses for the micro parameters.

Signed-off-by: Sonny Jiang <sonjiang@amd.com>
Reviewed-by: David (Ming Qiang) Wu <David.Wu3@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>