ENGR00281186 dmaengine: pxp: not fetch data from AS buffer if it's not used

ENGR00281186 dmaengine: pxp: not fetch data from AS buffer if it's not used

There's no explicit BIT in PxP register to disable AS buffer.
The right way to disable AS is to set ULC to a higher value than the LRC.
It has been done once in probe time, but not in run time.
This could cause one potential problem because even when ULC, LRC
and buffer address is set to 0 PxP will still fetch one pixel data from addr 0.
This patch fixes it by set ULC higher than LRC for AS if it's not used (i.e.,
no alpha-blending) at run time.

Signed-off-by: Robby Cai <R63905@freescale.com>