drm: Clean up the 1366x768 fixup codes

This is just a cleanup, no functional change.

The fixup code for 1366x768 in drm_mode_create_from_cmdline_mode() is
basically a copy of the existing code in drm_edid.c.  Make the latter
code public so that it can be called from the former function.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: http://patchwork.freedesktop.org/patch/msgid/20170117164329.10551-1-tiwai@suse.de


Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
[vsyrjala: include drm_crtc_internal.h to make sparse happy]
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>