i965: Implement ARB_compute_variable_group_size.

This patch adds the implementation of ARB_compute_variable_group_size
for i965. We do this by storing the local group size in a push constant.

Signed-off-by: Plamena Manolova <plamena.manolova@intel.com>
13 jobs for !1146 with compute_variable_group_size in 23 minutes and 32 seconds (queued for 9 seconds)
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