drm/amdgpu: use new flag to handle different firmware loading method

This patch introduces a new flag named "amdgpu_firmware_load_type" to
handle different firmware loading method. Since Vega10, there are
three ways to load firmware. It would be better to use a flag and a
fw_load_type kernel parameter to configure it.

Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>