m53evk: Force override of XSERVER setting

This really fixes the build failure due non-FSL kernel use with AMD
GPU. The commit b43b8470 didn't fix the issue, as we identified in our
autobuilder.

Change-Id: Ia56e9df03b316c78ad9c718f66afa70cffae45cd
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>