drm/amd/display: Clear dpcd_sink_ext_caps if not set

[WHY]
Some eDP panels' ext caps don't set initial values
and the value of dpcd_addr (0x317) is random.
It means that sometimes the eDP can be OLED, miniLED and etc,
and cause incorrect backlight control interface.

[HOW]
Add remove_sink_ext_caps to remove sink ext caps (HDR, OLED and etc)

Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Reviewed-by: Anthony Koo <anthony.koo@amd.com>
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Paul Hsieh <paul.hsieh@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>