etnaviv: Set SE.CLIP registers, add margins for scissor/clip registers

etnaviv: Set SE.CLIP registers, add margins for scissor/clip registers

This fixes rendering of full-screen quads (and other screen-filling
geometry, e.g. ioquake3 walls up-close) on gc3000. It should be a no-op
on other hardware.

- It looks like SE_CLIP registers were not set at all.
  I'm amazed that rendering worked without them. Emit them to
  avoid issues on gc3000.

- Define constants
  ETNA_SE_SCISSOR_MARGIN_RIGHT (0x1119)
  ETNA_SE_SCISSOR_MARGIN_BOTTOM (0x1111)
  ETNA_SE_CLIP_MARGIN_RIGHT (0xffff)
  ETNA_SE_CLIP_MARGIN_BOTTOM (0xffff)

  These demarcate the margin (fixp16) between the computed sizes and the
  value sent to the chip. I have set these to the numbers used by the
  Vivante driver for gc2000. I am not sure whether any old hardware was
  relying on the old numbers, or whether those were just a guess. But if
  so, these need to be moved to the _specs structure.

CC: <mesa-stable@lists.freedesktop.org>
Signed-off-by: Wladimir J. van der Laan <laanwj@gmail.com>
Acked-by: Christian Gmeiner <christian.gmeiner@gmail.com>