i965/clip: Fix brw_clip_unfilled.c/compute_offset's assembly.

i965/clip: Fix brw_clip_unfilled.c/compute_offset's assembly.

Due to the destination register width of 1 or 2, these instructions get
ExecSize 1 or 2.  But dir and offset (used as src0) are both registers
of width 4, violating the execsize >= width assertion.

I honestly don't think this could have ever worked.

Fixes Piglit's polygon-offset and polygon-mode-offset tests on Gen4-5.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=70441
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>