drm: Silence the compiler for drm_mode_get_hv_timings()

Since moving drm_crtc_get_hv_timings() into drm_modes.c, the compiler
has been able to get smarter and spots that drm_mode_copy() is trying to
preserve garbage from the stack.

Fixes: 196cd5d3 ("drm: s/drm_crtc_get_hv_timings/drm_mode_get_hv_timings/")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170126114409.9115-1-chris@chris-wilson.co.uk