drm/amdgpu: Refine the way to get atom bios

There are several ways to check out a ATOMBIOS. In previous codes, try
a new way to fetch out vbios/rom, until current vbios/rom is started with
0x55aa, then check if this vbios is ATOMBIOS. Now, try a new way to fetch
out vbios until all flags of ATOMBIOS are verified.

Signed-off-by: Ken Xue <Ken.Xue@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>