drm/amd/display: Remove duplicate null check

[WHAT & HOW]
The same display null check was a few lines above.

This fixes 1 DEADCODE issue reported by Coverity.

Reviewed-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>