drm: Constify adjusted_mode a bit

The DP link computation functions shouldn't modify the
adjusted_mode so make it const.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200319163844.22783-3-ville.syrjala@linux.intel.com


Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>