r300: respect output_semantic_index when writing colors

Right now we don't explicitly check it and expect that the output_semantic_index array is always ordered. Unfortunatelly, this is not the case since 74c02d99

Fixes renderign corruption in Amnesia: the Dark Descent.

cc: mesa-stable Closes: #6179 (closed) Signed-off-by: Pavel Ondračka pavel.ondracka@gmail.com