drm/amdgpu: added function to wait for PSP BL availability

reduced duplicate code

increased wait time for PSP BL readiness

Signed-off-by: John Clements <john.clements@amd.com>
Reviewed-by: Guchun Chen <guchun.chen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>