Add a proper check for feature support, and raise an invalid enum for
GL_CLAMP_VERTEX/FRAGMENT_COLOR unconditionally in core profiles, since
those enums were explicitly removed after the extension was promoted
to core functionality (not in the profile sense) with OpenGL 3.0.

This matches the behavior of the AMD closed source driver and fixes
GL45-CTS.gtf30.GL3Tests.half_float.half_float_textures.

Cc: "12.0 13.0" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>