mesa: Make validate_shader_target() non-static.
mesa: use _mesa_validate_shader_target() more frequently.

This patch replaces code in _mesa_new_shader() and delete_shader_cb()
that checks the type of a shader with calls to
_mesa_validate_shader_target().  This has two advantages: it allows
for a more thorough check (since _mesa_validate_shader_target()
doesn't permit shader targets that aren't supported by the back-end),
and it reduces the amount of code that will need to be modified when
adding new shader stages.