broadcom/vc4: Mark BOs as purgeable when they enter the BO cache

broadcom/vc4: Mark BOs as purgeable when they enter the BO cache

This patch makes use of the DRM_IOCTL_VC4_GEM_MADVISE ioctl to mark all
BOs placed in the mesa BO cache as purgeable so that the system can
reclaim this memory under memory pressure.

v2:
- Removed BOs from the cache when they've been purged by the kernel
- Check whether the madvise ioctl is supported or not before using it

v3: Don't walk the whole list when we find a busy BO (by anholt, acked by
    Boris)

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Reviewed-by: Eric Anholt <eric@anholt.net>