This helper was only called from _mesa_BindTextureUnit().  It's simpler
to just inline it.

The error check / code / message in the helper was incorrect.  It was
written for glBindTextures(), not glBindTextureUnit().  The correct
error for a bad texture unit number is GL_INVALID_VALUE.  The error
message now reports the unit number rather than a GL_TEXTUREi enum.

Fixes a failure in piglit's arb_direct_state_access-bind-texture-unit test.

Reviewed-by: Tapani Pälli <tapani.palli@intel.com>