i965/vs: Add support for ir_tg4

Pretty much the same as the FS case. Channel select goes in the header,
post-sampling swizzle only does the 0/1 cases.

V2: Less mangling.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
i965: Enable ARB_texture_gather on Gen7

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>