drm/amd/display: Refactor and fix link_dpms info

drm/amd/display: Refactor and fix link_dpms info

[Why]
get_ext_hdmi_settings contained a copy-paste error, assigning
both reg_num_6g and reg_num from dp0_ext_hdmi_6g_reg_num.

[How]
* Correctly assign reg_num from dp0_ext_hdmi_reg_num.
* Refactor and clean the function.

Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Signed-off-by: Dominik Kaszewski <dominik.kaszewski@amd.com>
Signed-off-by: Ray Wu <ray.wu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>