drm/amdgpu: Report vram vendor with sysfs (v3)

The vram vendor can be found as a separate sysfs file at:
/sys/class/drm/card[X]/device/mem_info_vram_vendor
The vram vendor is displayed as a string value.

v2: Use correct bit masking, and cache vram_vendor in gmc
v3: Drop unused functions for vram width, type, and vendor

Signed-off-by: Ori Messinger <ori.messinger@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>