drm/amdgpu: fix spelling mistake "gateing" -> "gating"

There is a spelling mistake in a DRM_INFO message. Fix it.

Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>