etnaviv: flush all pending contexts when accessing a resource with the CPU

etnaviv: flush all pending contexts when accessing a resource with the CPU

When setting up a transfer to a resource, all contexts where the resource
is pending must be flushed. Otherwise a write transfer might be started
in the current context before all contexts that access the resource in
shared (read) mode have been executed.

Fixes: 64813541d575 (etnaviv: fix resource usage tracking across
different pipe_context's)
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Tested-By: Guido Günther <agx@sigxcpu.org>