drm/amdgpu: enable dynamic GECC support (v2)

Dynamic GECC allows user to specify GECC enablement
status, which will take effect in next boot cycle.

v2: initialize boot_cfg to 0xFF

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>