mesa: use same is_color_attachment trick to discern error cases

mesa: use same is_color_attachment trick to discern error cases

All the other calls to retrieve the attachment have been covered except
this one - return the proper error for attachment points that are valid
enums but out of bound for the driver.

Fixes GL45-CTS.geometry_shader.layered_fbo.fb_texture_invalid_attachment

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>