drm: xlnx: pl_disp: Enable early callback

drm: xlnx: pl_disp: Enable early callback

Patch adds support for enabling early callback. This enables the
user space applications like kmssink plugin that queue up only one
buffer at a time to not drop any frames due to timeout receiving
vblank event from the driver.
The side effect of enabling this behavior is that the buffer whose
callback function has been called may actually still be scanning
out.

Signed-off-by: Vishal Sagar <vishal.sagar@xilinx.com>
Reviewed-by: Hyun Kwon <hyun.kwon@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>