i965: Move ClipPlanesEnabled state to VS cache key.

i965: Move ClipPlanesEnabled state to VS cache key.

Previous to this patch, setup_uniform_clipplane_values() was setting
up clip plane uniforms based on ctx->Transform.ClipPlanesEnabled, a
piece of state not stored in the vertex shader cache key.  As a
result, a change to this piece of state might not trigger a necessary
vertex shader recompile.

The patch adds a field to the vertex shader cache key,
userclip_planes_enabled, to store the current value of
ctx->Transform.ClipPlanesEnabled.  Also, it changes
setup_uniform_clipplane_values() to read from this new field, so that
it's manifestly clear that the vertex shader isn't depending on state
not stored in the cache key.

Note: when the vertex shader uses gl_ClipDistance, the VS backend
doesn't need to know which clip planes are in use, so we leave the
field as zero in that case to avoid unnecessary recompiles.

Fixes Piglit test vs-clip-vertex-enables.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>