drm/amd/display: Remove unnecessary NULL check

[Why]
new_crtc_state is already dereferenced earlier in the function

[How]
Remove the check

Signed-off-by: Eryk Brol <eryk.brol@amd.com>
Acked-by: Bindu Ramamurthy <bindu.r@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>