drm/edid: abstract debugfs override EDID set/reset

drm/edid: abstract debugfs override EDID set/reset

Add functions drm_edid_override_set() and drm_edid_override_reset() to
support "edid_override" connector debugfs, and to hide the details about
it in drm_edid.c. No functional changes at this time.

Also note in the connector.override_edid flag kernel-doc that this is
only supposed to be modified by the code doing debugfs EDID override
handling. Currently, it is still being modified by amdgpu in
create_eml_sink() and handle_edid_mgmt() for reasons unknown. This was
added in commit 569de45cb14d ("drm/amd/dc: Add dc display driver (v2)")
and later moved to amdgpu_dm.c in commit 0dd8720d15a4 ("drm/amd/display:
Merge amdgpu_dm_types and amdgpu_dm").

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/8f6b4001630cafac5f44aa5913429ac9979743d2.1656494768.git.jani.nikula@intel.com