linux/build.sh: re-enable copying pocket reform dts (disabled in 'initial support for meson-g12b-bananapi-cm4-mnt-reform2')

linux/patches/imx8mq-mnt-reform2/0005-imx8mq-import-HDMI-driver-and-make-DCSS-compatible-w.patch: move cdn-mhdp stuff into cdns subdirectory

linux/patches/0005-imx8mq-import-HDMI-driver-and-make-DCSS-compatible-w.patch: let DRM_CDNS_HDMI select DRM_DISPLAY_HDMI_HELPER

set SOURCE_DATE_EPOCH based on last git commit and use it for d/changelog and embed it in the kernel version