drm/amdgpu: enlarge the VBIOS binary size limit

Some chips have a larger VBIOS file so raise the size limit to support
the flashing tool.

Signed-off-by: Shiwu Zhang <shiwu.zhang@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>