freedreno/a6xx: Support y-inverted blits

freedreno/a6xx: Support y-inverted blits

The src coordinates are s24.8. For an inverted blit that ends at y=0
we need to program -1 for sy2, so we need to handle negative values
correctly.

Fixes

  dEQP-GLES3.functional.fbo.blit.rect.nearest_consistency_mag_reverse_dst_y
  dEQP-GLES3.functional.fbo.blit.rect.nearest_consistency_min_reverse_dst_y
  dEQP-GLES3.functional.fbo.blit.rect.nearest_consistency_min_reverse_src_y
  dEQP-GLES3.functional.fbo.invalidate.sub.unbind_blit_color
  dEQP-GLES3.functional.fbo.invalidate.whole.unbind_blit_color

Reviewed-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Kristian H. Kristensen <hoegsberg@chromium.org>