drm/amd/display: explicitly set is_dsc_supported to false before use

When UBSAN is enabled a case is shown on unplugging the display that
this variable hasn't been initialized by `update_dsc_caps`, presumably
when the display was unplugged it wasn't copied from the DPCD.

Link: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1956497
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>