drm/amd/display: Only actually breakpoint if DEBUG_KERNEL_DC is enabled

To match previous behavior and to not hang the kernel if someone
accidentally builds with KGDB enabled.

Fixes: 1aad7078 ("drm/amd/display: Make BREAK_TO_DEBUGGER() a debug print")

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>