drm: Check if the allocation has succeeded before dereferencing newmode

We allocate memory in drm_display_mode_from_vic_index() and use it
without checking the pointer is valid. Fix that.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>