drm/i915/gvt: unify lri cmd handler and mmio handlers

for common ctxs and commands in workload, call mmio handlers for value
checking and command patching for registers with command fix write flag
set.

after calling mmio handlers, resotre old vreg values so as to keep vreg
unchanged after command scanning.

Note:
for regs in below catigories,
a. in ctx and save-restore list, and is inhibit ctx
b. in mocs
after cmd scanning, do not restore old vreg values, but update vreg as
what has been implemented before this patch.

Cc: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20201223034632.17416-1-yan.y.zhao@intel.com
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>