drm/amdgpu: disable DRAM memory training when GECC is enabled

GECC and G6 mem training are mutually exclusive
functionalities. VBIOS/PSP will set the flag
(BOOT_CFG_FEATURE_TWO_STAGE_DRAM_TRAINING) in
runtime database to indicate whether dram memory
training need to be disabled or not.

For Navi1x families, two stage mem training is always
enabled.

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: John Clements <john.clements@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>