drm/amd/display: Fix use before NULL check in validate_timing

Found by smatch:
drivers/gpu/drm/amd/amdgpu/../display/dc/dce110/dce110_timing_generator.c:1124
dce110_timing_generator_validate_timing() warn: variable dereferenced
before check 'timing' (see line 1116)

Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>