drm/amdgpu: Add VBIOS flags

Instead of read_bios, use get_bios_flags to get various options around
reading VBIOS.

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