ARM: bcm2835: Enable missing CMA settings for VC4 driver

Currently bcm2835_defconfig has CMA disabled which makes the
HDMI output on a Raspberry Pi 1 stop working during boot:

    fb: switching to vc4drmfb from simple
    Console: switching to colour dummy device 80x30
    [drm] Initialized vc4 0.0.0 20140616 for soc:gpu on minor 0
    [drm] Supports vblank timestamp caching Rev 2 (21.10.2013).
    [drm] Driver supports precise vblank timestamp query.
    vc4-drm soc:gpu: failed to allocate buffer with size 9216000
    vc4-drm soc:gpu: Failed to set initial hw configuration.

So enable CMA and DMA_CMA in bcm2835_defconfig.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Fixes: 4400d9ac ("ARM: bcm2835: Enable the VC4 graphics driver in the defconfig")
Reviewed-by: Eric Anholt <eric@anholt.net>