drm/amdgpu/atomfirmware: fix parsing of vram_info

v3.x changed the how vram width was encoded.  The previous
implementation actually worked correctly for most boards.
Fix the implementation to work correctly everywhere.

This fixes the vram width reported in the kernel log on
some boards.

Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org