drm/amd/display: fix spelling mistake: "Usupported" -> "Unsupported"

Trivial fix to spelling mistake in DRM_ERROR error message text

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