msm: mdss: Add support to retain AD RAMs

Modify the MDP_CBCR bits to retain the AD RAMs through idle power
collapse(IPC).

Bits setting when device enters IPC:
  - MEM_CORE_ON: 1
  - MEM_PERIPH_ON: 0
  - MEM_PERIPH_OFF: 1
Bits setting when device exits the IPC:
  - MEM_CORE_ON: 1
  - MEM_PERIPH_ON: 1
  - MEM_PERIPH_OFF: 0

Change-Id: I10749312d905eb4629e94e396b1604667738e84f
Signed-off-by: Ping Li <pingli@codeaurora.org>