dtoverlays: Rework vc4-kms-dpi overlays to remove duplication
Removes all the common panel, dpi, and backlight configuration
from the individual vc4-kms-dpi-* files into vc4-kms-dpi.dtsi.
Creates a new vc4-kms-dpi-panel-overlay.dts for preconfigured
panels, with overrides to enable the different panel configurations.
Deprecates vc4-kms-dpi-at056tn53v1 as superceded by vc4-kms-dpi-panel.
vc4-kms-kippah-7inch not deprecated for now as it is likely to be
in wider use than at056tn53v1.
Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>