nir: Make nir_lower_samplers take a gl_shader_stage, not a gl_program *.

nir: Make nir_lower_samplers take a gl_shader_stage, not a gl_program *.

We don't actually need a gl_program struct.  We only used it to
translate prog->Target (i.e. GL_VERTEX_PROGRAM) to the gl_shader_stage
(i.e. MESA_SHADER_VERTEX).  We may as well just pass that.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>