Fix black screen after resume from hibernate.

Fix black screen after resume from hibernate.

The root cause for the black screen and system lock up is
caused by not recovering the SVGA ID register after hibernation.
Incorrect ID register value will invalidate the FIFO memory start
register, and driver will not retrieve correct FIFO memory start
address and the busy read of svga FIFO sync register will lock up
the whole system.

Currently SVGA Xorg driver does not have a kernel module to handle
the power management event, but Xorg will call driver provided
LeaveVT before shutting down system and call EnterVT after resuming
system from hibernation, so these two callback functions are good
entry points to save and restore the ID register value. This patch
saves the ID register value in LeaveVT and restores the value to
SVGA ID register in EnterVT.