mesa: move max texture image unit constants to gl_program_constants

mesa: move max texture image unit constants to gl_program_constants

Const.MaxTextureImageUnits -> Const.FragmentProgram.MaxTextureImageUnits
Const.MaxVertexTextureImageUnits -> Const.VertexProgram.MaxTextureImageUnits
etc.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Eric Anholt <eric@anholt.net>