mesa: Make GenerateMipmap check the target before finding an object.

mesa: Make GenerateMipmap check the target before finding an object.

If glGenerateMipmap was called with a bogus target, then it would
pass that to _mesa_get_current_tex_object(), which would raise a
_mesa_problem() telling people to file bugs.  We'd then do the
proper error checking, raise an error, and bail.

Doing the check first avoids the _mesa_problem().  The DSA variant
doesn't take a target parameter, so we leave the target validation
exactly as it was in that case.

Fixes one dEQP GLES2 test:
dEQP-GLES2.functional.negative_api.texture.generatemipmap.invalid_target.

v2: Rebase on Antia's recent patch to this area.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Brian Paul <brianp@vmware.com> [v1]
Reviewed-by: Matt Turner <mattst88@gmail.com>