drm/ingenic: Use the highest possible DMA burst size

Until now, when running at the maximum resolution of 1280x720 at 32bpp
on the JZ4770 SoC the output was garbled, the X/Y position of the
top-left corner of the framebuffer warping to a random position with
the whole image being offset accordingly, every time a new frame was
being submitted.

This problem can be eliminated by using a bigger burst size for the DMA.

Set in each soc_info structure the maximum burst size supported by the
corresponding SoC, and use it in the driver.

Set the new value using regmap_update_bits() instead of
regmap_set_bits(), since we do want to override the old value of the
burst size. (Note that regmap_set_bits() wasn't really valid before for
the same reason, but it never seemed to be a problem).

Cc: <[email protected]>
Fixes: 90b86fcc47b4 ("DRM: Add KMS driver for the Ingenic JZ47xx SoCs")
Signed-off-by: Paul Cercueil <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
Acked-by: Sam Ravnborg <[email protected]>
Tested-by: Christophe Branchereau <[email protected]>

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