mesa: fix GL_COLOR_SUM enum for drivers without ARB_vertex_program

mesa: fix GL_COLOR_SUM enum for drivers without ARB_vertex_program

Commit c13970808 (mesa: GL_EXT_secondary_color is not optional) changed

CHECK_EXTENSION2(EXT_secondary_color, ARB_vetex_program, cap)

to

CHECK_EXTENSION(ARB_vertex_program, cap)

However CHECK_EXTENSION2 checks that either extension is available, not
both. Remove the extension check entirely since the intent was for it to
always be enabled.

v2: Fix glGet*(GL_COLOR_SUM) too.  Suggested by Ian.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Cc: 9.2 10.0 <mesa-stable@lists.freedesktop.org>