drm: xlnx: zynqmp_dpsub: switch to ->edid_read callback

Prefer using the struct drm_edid based callback and functions.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/549694273b57e7275de01daf8ce60579121998d4.1706038510.git.jani.nikula@intel.com