i965: Only turn on ARB_compute_shader if we can write registers.

i965: Only turn on ARB_compute_shader if we can write registers.

Compute shaders require reconfiguring the L3 for shared local memory
support.  We have to be able to write the L3 registers to do that.

This effectively turns off compute shaders prior to Kernel 4.2.

(Previously, the extension enable was in an API_OPENGL_CORE conditional.
However, that isn't necessary - core Mesa extension handling already
restricts it properly.  I've moved it out in this patch.)

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>