i915: disable GEN6_MBCTL write in gen6_init_clock_gating

This write came from Linux commit b4ae3f22d238 which has been implicated
in Sandy Bridge power consumption issues (albeit under different
conditions on Linux). Disabling it restores normal power consumption on
my Sandy Bridge laptop (Thinkpad X220).

PR: 207889
Reviewed by: cem, dumbbell
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D5665