drm/amdgpu/gmc11: implement get_vbios_fb_size()

Implement get_vbios_fb_size() so we can properly reserve
the vbios splash screen to avoid potential artifacts on the
screen during the transition from the pre-OS console to the
OS console.

Acked-by: Sunil Khatri <sunil.khatri@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>