video: da8xx-fb: fix flicker due to 1 frame delay in updated frame

Flicker/tearing effect is observed with current FB driver.
Issue is because of 2 active DMA channels ping ponging among them
along with usage of 2 DDR ping pong buffers in driver. Application
unaware of active DMA channel keeps updating frame being displayed,
this leads to tearing effect.
Below steps describes the issue:
1)Initially assume both buffers FB0 and FB1 are programmed for buffer-0.
2)On EOF0: Program FB0 for buffer-1, indicate(wake up) application
 to fill up buffer-0. As FB1 is active and continues to DMA buffer-0
(which is being filled), leading to tearing/flickering issue.
3)On EOF1: Program FB1 for buffer-0, indicate(wake up) application to
 fill up buffer-1. As FB0 is active and continues to DMA buffer-1(which
 is being filled), leading to tearing/flickering issue.
4)On EOF0: Program FB0 for buffer-1, indicate(wake up) application to
fill up buffer-0. As FB1 is active and continues to DMA buffer-0(which is
being filled), leading to tearing/flickering issue.
...
Above steps depict that issue is because of 1 frame delay in frame
panned by application.

Patch fixes the issue by keeping track free DMA channel and configures
it in drivers PAN callback so that panned frame from application gets
displayed in next frame period.

Wiki below describes the issue in detail and it also has link to
application with which issue can be reproduced.
http://processors.wiki.ti.com/index.php/DA8xx_LCDC_Linux_FB_FAQs

Signed-off-by: Nellutla, Aditya <aditya.n@ti.com>
Signed-off-by: Manjunathappa, Prakash <prakash.pm@ti.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>

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