i915: make array flex_regs static const

Don't populate the read-only array flex_regs on the stack but
instead it static const. Also makes the object code a little smaller.

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220112223435.949071-1-colin.i.king@gmail.com