i965: use _Shader to get fragment program when updating surface state

i965: use _Shader to get fragment program when updating surface state

Atomic counters and Images were using ctx::Shader that does not take in
to account program pipeline changes, ctx::_Shader must be used for SSO to
work. Commit c0347705 already changed ubo's to use this.

Fixes failures seen with following Piglit test:
	arb_separate_shader_object-atomic-counter

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Cc: "11.0 11.1" <mesa-stable@lists.freedesktop.org>