linux-postmarketos-allwinner: restore CMA size

This partly reverts the commit mentioned below. When disabling CMA
I thought that libcamera was the only user of it, which is only true
for userspace, though.
In fact CMA is used and often even required by the kernel for HW video
decoders, display engines and other hardware that might not have a
MMU.

Thus restore the previously established value.

Fixes: 196ece02 (linux-postmarketos-allwinner: enable CONFIG_UDMABUF (MR 5585))

device/main/linux-postmarketos-allwinner/APKBUILD

@@ -81,7 +81,7 @@ package() {
 }
 sha512sums="
 c6081383741fe883d8f6e223caa45d7ecf179f4cd0c759432bbbb2e43603088840393a0cf58735ab5dda4dbf5e2655d8c9b053a74b029ac4768c200ff6ca6f65  linux-postmarketos-allwinner-orange-pi-6.9-20240721-2345.tar.gz
-e3daa0fdca1a522bd1ffcfede8e387be594cf9f71f9a04b11dba66bfea04ea216a216c55deeb4cff630c5c55f374a9114ff210ede1c0511bccff6ff4007eb5be  config-postmarketos-allwinner.aarch64
+3281e69061af98020adf412773a187cb3245b26e0dc22c29a08f37c64360e8ad23a5ea2c4cc19365e2c2ed07d7aae0d63e9e116a9f0badb25584dfe763f3476c  config-postmarketos-allwinner.aarch64
 0011646badbea59ecc670667252d0f6b9fe84c05e43cd3583f7730de526d072f29f47100f65d17dbf84d1284ed6b6cae8eb9d7b6a8df33a178cdfa376402b2e1  config-postmarketos-allwinner.armv7
 a1fe1a21c34062eb92b6a4a9d2ef330d0e745859af283ccf05fc899ec263f9d452c1399c29620a0e4c748eca4b0c13645668b2fc0dc72f5703426d472f1294be  0001-dts-add-dontbeevil-pinephone-devkit.patch
 0ef26b34fa834c119ebff6daa27b1dd3fac27448d6a40c0b2022a00c239dad0ef3e2b3ab3f9eec5460f921c9c6ea21db360adc175a7e658d5c34c5d0aacc498f  0002-dts-add-pinetab-dev-old-display-panel.patch

device/main/linux-postmarketos-allwinner/config-postmarketos-allwinner.aarch64

@@ -6581,7 +6581,7 @@ CONFIG_DMA_CMA=y
 #
 # Default contiguous memory area size:
 #
-CONFIG_CMA_SIZE_MBYTES=0
+CONFIG_CMA_SIZE_MBYTES=256
 CONFIG_CMA_SIZE_SEL_MBYTES=y
 # CONFIG_CMA_SIZE_SEL_PERCENTAGE is not set
 # CONFIG_CMA_SIZE_SEL_MIN is not set