ARM: bcm2835_defconfig: Increase CMA for VC4

The VC4 needs more memory than the default setting:

    cma: cma_alloc: alloc failed, req-size: 4096 pages, ret: -12
    vc4-drm soc:gpu: failed to allocate buffer with size 16777216

So increase the value to 32 MB and fix this issue.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Eric Anholt <eric@anholt.net>