i965/vs: Move compute_clip_distance() out of emit_urb_writes().

i965/vs: Move compute_clip_distance() out of emit_urb_writes().

Legacy user clipping (using gl_Position or gl_ClipVertex) is handled by
turning those into the modern gl_ClipDistance equivalents.

This is unnecessary in Core Profile: if user clipping is enabled, but
the shader doesn't write the corresponding gl_ClipDistance entry,
results are undefined.  Hence, it is also unnecessary for geometry
shaders.

This patch moves the call up to run_vs().  This is equivalent for VS,
but removes the need to pass clip distances into emit_urb_writes().

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>