drm/i915/gvt: rename F_IN_CTX flag to F_SR_IN_CTX

F_IN_CTX is an inaccurate flag name, because people may wrongly think all
MMIOs in context image are with this flag. But actually, this flag is only
for MMIOs both in GVT's save-restore list and in hardare logical
context's image.

Reviewed-by: Zhenyu Wang <zhenyuw@linux.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/20200811060944.3039-1-yan.y.zhao@intel.com