drm_get_cea_aspect_ratio() is not used outside drm_edid.c.
Make it static.

Cc: Wayne Lin <waynelin@amd.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191004141914.20600-1-ville.syrjala@linux.intel.comReviewed-by: Uma Shankar <uma.shankar@intel.com>