i915: fix wrong rendering of gl_PointSize on Pineview

i915: fix wrong rendering of gl_PointSize on Pineview

The current code would ignore the point size specified by gl_PointSize
builtin variable in vertex shader on Pineview. This patch servers as
fixing that.

This patch fixes the following issues on Pineview:
webglc: https://cvs.khronos.org/svn/repos/registry/trunk/public/webgl/sdk/tests/conformance/rendering/point-size.html
piglit: glsl-vs-point-size

NOTE: This is a candidate for stable release branches.

v2: pick Eric's nice tip for fixing this issue in hardware rendering.
v3: the last arg of EMIT_ATTR specify the size in _byte_. (Eric)

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>