mesa context: generate an error for uninstalled context functions

mesa context: generate an error for uninstalled context functions

For 'non-legacy' contexts we will want to generate an error
if an uninstalled function is called.

The effect of this change will be that we can avoid installing
legacy functions, and they will then generate an error as
needed for deprecated functions in GL >= 3.1.

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>