drm/edid: make drm_edid_are_equal() static

drm_edid_are_equal() is only used within drm_edid.c. Make it static. Do
not encourage more uses of struct edid.

Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/aa79be9a5d0b08c71b82b86b5a8ff0f332e13c6a.1713259151.git.jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>