i965: allow for nonconsecutive elements of gl_ClipDistance to be enabled.

i965: allow for nonconsecutive elements of gl_ClipDistance to be enabled.

When using user-defined clipping planes, the i965 driver compacts the
array of clipping planes so that disabled clipping planes do not
appear in it--this saves precious push constant space and makes it
easier to generate the pre-GEN6 clip program.  As a result, when
enabling clipping planes in GEN6+ hardware, we always enable clipping
planes 0 through n-1 (where n is the number of clipping planes
enabled), regardless of which clipping planes the user actually
requested.

However, we can't do this when using gl_ClipDistance, because it would
be prohibitively complex to compact the gl_ClipDistance array inside
the user-supplied vertex shader.  So, when enabling clipping planes in
GEN6+ hardware, if gl_ClipDistance is in use, we need to pass the
user-supplied enable flags directly through to the hardware rather
than just enabling the first n planes.

Fixes Piglit test vs-clip-distance-enables.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>