i965: Allow clip distances to be read back in fragment shaders.

i965: Allow clip distances to be read back in fragment shaders.

Previously, we conditionally set up the SF pipline stage with a
urb_entry_read_offset of 2 when clipping was in use, and 1 otherwise,
causing the clip distance VUE slots to be skipped if present.  This
was an extremely minor savings (it saved the SF unit from reading 2
vec4s out of the URB, but it didn't affect any computation, since we
only instruct the SF unit to perform interpolation on VUE slots that
are actually used by the fragment shader).

GLSL 1.30 requires an interpolated version of gl_ClipDistance to be
available for reading in the fragment shader, so we need the SF's
urb_entry_read_offset to be 1 when the fragment shader reads from
gl_ClipDistance.

This patch just unconditionally sets the urb_entry_read_offset to 1 in
all cases; this is sufficient to make gl_ClipDistance available to the
fragment shader when it is needed, and the performance loss should be
negligible when it isn't.

Reviewed-by: Eric Anholt <eric@anholt.net>