drm/radeon: fix a possible null pointer dereference

[ Upstream commit 2c1fe3c4 ]

In radeon_tv_get_modes(), the return value of drm_cvt_mode()
is assigned to mode, which will lead to a NULL pointer
dereference on failure of drm_cvt_mode(). Add a check to
avoid null point dereference.

Signed-off-by: Ma Ke <make_ruc2021@163.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>