i915g: fix incorrect gl_FragCoord value

i915g: fix incorrect gl_FragCoord value

On Intel Pineview M hardware, the i915 gallium driver doesn't output
the correct gl_FragCoord. It seems to always have an X coord of 0.0
and a Y coord of the window's height in pixels, e.g. 600.0f or such.

I believe this is a regression caused in part by this commit:
afa035031ff9e0c07a2297d864e46c76f7bfff58

The old behavior used the output at index zero, while the new behavior
uses actual zeroes. In the case of gl_FragCoord the output at index
zero happened to be the correct one, so the behavior appeared correct
although the code already had a bug.

Fixed by checking for I915_SEMANTIC_POS when setting up texCoords. If
the generic_mapping is I915_SEMANTIC_POS, look for the
TGSI_SEMANTIC_POSITION instead of a TGSI_SEMANTIC_GENERIC output.

https://bugs.freedesktop.org/show_bug.cgi?id=97477

Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
Tested-by: Stéphane Marchesin <marcheu@chromium.org>