drm/amdgpu/atomfirmware: simplify the interface to get vram info

fetch both the vram type and width in one function call.  This
avoids having to parse the same data table twice to get the two
pieces of data.

Reviewed-by: Xiaojie Yuan <xiaojie.yuan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>