drm/amdgpu: reserve more memory for MES runtime DRAM

This patch fixes a MES firmware boot failure issue
when backdoor loading the MES firmware.

MES firmware runtime DRAM size is changed to 512k,
the driver needs to reserve this amount of memory in
FB, otherwise adjacent memory will be overwritten by
the MES firmware startup code.

Signed-off-by: Tim Huang <Tim.Huang@amd.com>
Reviewed-by: Yifan Zhang <yifan1.zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>