drm/i915: Add function to clear scanout flag for vmas

Currently frontbuffer tracking code is directly iterating over object vmas
and clearing scanout flags for them. Add function to clear scanout flag for
vmas and use it from frontbuffer tracking code.

v2: describe function parameter.

Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230727064142.751976-5-jouni.hogander@intel.com