drm/amd/display: Don't invoke kgdb_breakpoint() unconditionally

ASSERT_CRITICAL() invokes kgdb_breakpoint() whenever either
CONFIG_KGDB or CONFIG_HAVE_KGDB is set.  This, however, may lead to a
kernel panic when no kdb stuff is attached, since the
kgdb_breakpoint() call issues INT3.  It's nothing but a surprise for
normal end-users.

For avoiding the pitfall, make the kgdb_breakpoint() call only when
CONFIG_DEBUG_KERNEL_DC is set.

https://bugzilla.opensuse.org/show_bug.cgi?id=1177973


Cc: <stable@vger.kernel.org>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>