rename etna_context to etna_cmd_stream

etna_cmd_stream is a better naming as it only represents
a stream of gpu commands to interact with.

Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>
rename etnaviv_context.c to etnaviv_cmd_stream.c

Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>