intel/i965: make gen_device_info mutable

intel/i965: make gen_device_info mutable

Make gen_device_info a mutable structure so we can update the fields that
can be refined by querying the kernel (like subslices and EU numbers).

This patch does not make any functional change, it just makes
gen_get_device_info() fill a structure rather than returning a const
pointer.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>