drm/amd: remove possible condition with no effect (if == else)

This patch fix cocci warning:
drivers/gpu/drm/amd/display/dc/core/dc.c:3335:2-4: WARNING:
possible condition with no effect (if == else).

Signed-off-by: Bernard Zhao <bernard@vivo.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>