i965: relax brw_texture_offset assert

Some texturing ops are about to have nonconstant offset support; the
offset in the header in these cases should be zero.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
i965/fs: add support for gather4 with nonconstant offsets

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
i965/vs: add support for gather4 with nonconstant offsets

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