drm/i915/gvt: add/modify interfaces for flag F_CMD_ACCESS

flag F_CMD_ACCESS represents whether an MMIO is able to be accessed by
GPU commands.
In this patch,
1. add interface to set this flag
2. rename intel_gvt_mmio_is_cmd_access() to
intel_gvt_mmio_is_cmd_accessible() and update its description message.

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/20200811070233.3387-1-yan.y.zhao@intel.com