ENGR00279413 pxp/v4l2: get the right framebuffer start address at run time

ENGR00279413 pxp/v4l2: get the right framebuffer start address at run time

Previously the driver gets the framebuffer start address at probe time.
But this address might be changed if the framebuffer drivers re-allocate
the frame buffers due to the application changes the yres_virtual.
As a result, some garbage data can be observed on display.
This patch adjusts the way to detect the start address at run time to
fix this problem.

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