i965: Specialize SURFACE_STATE creation for shader time.

i965: Specialize SURFACE_STATE creation for shader time.

This is basically a copy and paste of gen7_create_constant_surface, but
with the parameters filled in to offer a simpler interface.

It will diverge shortly.

I didn't bother adding it to the vtable for now since shader time is only
exposed on Gen7+.

v2: Replace tabs in the new code (by anholt)
    Add back dropped memset() and add a comment about HSW channel selects.

NOTE: This is a candidate for the 9.1 branch.
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Paul Berry <stereotype441@gmail.com>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>