r600g: fix up vs export handling

r600g: fix up vs export handling

Certain attributes (position, psize, etc.) don't
count as params; they are handled separately by the hw.
However, the VS is required to export at least one param
and r600_shader_from_tgsi() takes care of adding a dummy
export if there is none.  Make sure the VS param export
count in the SPI properly accounts for this.

Note: This is a candidate for the 7.11 branch.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>