drm/amd/display: add debugfs for allow_edp_hotplug_detection

[Why] within dc_link_detect, edp edid is read only for the first time
and saved. edid will not be read after the first time read. to run edp
edid read test, need read edp edid for each dc_link_detect. dc->config
flag allow_edp_hotplug_detection could be used for edp edid test.

[How] add debugfs for dc->config.allow_edp_hotplug_detection

Acked-by: Stylon Wang <stylon.wang@amd.com>
Signed-off-by: Hersen Wu <hersenxs.wu@amd.com>
Reviewed-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>