On KB, KV, CZ we should read the vram width from integrated system
table, if we can. The NOOFCHAN in MC_SHARED_CHMAP is not accurate.

With this change we can enable two 4k displays on CZ again. This use
case was broken sometime in January when we started looking at
vram_width for bandwidth calculations instead of hardcoding this value.

v2:
  Return 0 if integrated system info table is not available.
Tested-by: NRoman Li <roman.li@amd.com>
Signed-off-by: NHarry Wentland <harry.wentland@amd.com>
Reviewed-by: NAlex Deucher <alexander.deucher@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>