amd/display/dc: remove repeating expression

Identify issues that arise by using the tests/doubletest.cocci
semantic patch. Need to remove duplicate expression in if statement.

Signed-off-by: Wang Ming <machel@vivo.com>
Reviewed-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>