gallium/mesa: enhance PIPE_CAP_CLIP_PLANES to support override number

gallium/mesa: enhance PIPE_CAP_CLIP_PLANES to support override number

There exists hardware intel gen4 specifically that has only 6 clip planes
but supports GLSL 1.30. This enhances the CAP so that the current values
of 0,1 remain the same, but giving it a larger number will override the
max.

This allows the gen4 intel to set this to 6 and leave everyone else alone.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14344>