hwc: Bandwidth check upgrade.

Use the low and high bandwidth values exported from driver per target
Use the low bandwidth value when the system has at least 1 video
mode panel, else use the high bandwidth value.
Newer targets could have bandwidth exceeding what an uint32 could
hold, so change the bytes read to be in gigs.
Do not use the framebuffer handle to get size, since the handle may
be null.

Change-Id: If3124ddc33334eba80d34ab7d8fc64d1b46697ce
2 files changed