drm/amdgpu/display: Fix spelling mistake "supporing" -> "supporting"

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

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>