drm/i915/gvt: Fix two CFL MMIO handling caused by regression.

drm/i915/gvt: Fix two CFL MMIO handling caused by regression.

D_CFL was incorrectly removed for:
GAMT_CHKN_BIT_REG
GEN9_CTX_PREEMPT_REG

V2: Update commit message.
V3: Rebase and split Fixes and mis-handled MMIO.

Fixes: c8b1fe202c33 (drm/i915/gvt: replaced register address with name)
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Colin Xu <colin.xu@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20200601030638.16002-1-colin.xu@intel.com