drm/amd/display: remove useless else if

Fix the following coccinelle report:
drivers/gpu/drm/amd/display/dc/dcn302/dcn302_resource.c:1357:6-8:
WARNING: possible condition with no effect (if == else)

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>