rs: set pipe_offset
rs: update RS_WINDOW_SIZE for two pixel pipes

on a vivante gpu with only one pixel pipe the following gets used:

  0x08010588, /* LOAD_STATE (1) Base: 0x01620 Size: 1 Fixp: 0 */
  0x010001c0, /*   [01620] RS.WINDOW_SIZE := HEIGHT=256,WIDTH=448 */

with two pixel pipes the following gets used:

  0x008001c0, /*   [01620] RS.WINDOW_SIZE := HEIGHT=128,WIDTH=448 */
  0x080205c0, /* LOAD_STATE (1) Base: 0x01700 Size: 2 Fixp: 0 */
  0x00000000, /*   [01700] RS.PIPE[0].OFFSET := X=0,Y=0 */
  0x00800000, /*   [01704] RS.PIPE[1].OFFSET := X=0,Y=128 */
  0x00000000, /* PAD */
make etna_ctx available in etna_compile_rs_state(..)

Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>
rs: do computation of RS_WINDOW_SIZE in etna_compile_rs_state

Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Merge pull request #19 from austriancoder/master

gc2000: fix RS_PIPE_OFFSET and RS_WINDOW_SIZE