drm/amdgpu: fix regression in vbios reservation handling on headless

We need to move the check under the non-headless case, otherwise
we always reserve the VGA save size.

Fixes: 157fe68d

 ("drm/amdgpu: fix size calculation with stolen vga memory")
Reviewed-by: Guchun Chen <guchun.chen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>