i965/Gen4: Zero extra coordinates for ir_tex

We always emit U,V,R coordinates for this message, but the sampler gets
very angry if pass garbage in the R coordinate for at least some
texture formats.

Fill the remaining coordinates with zero instead.

Fixes broken rendering on GM45 in Source games.

NOTE: This is a candidate for stable branches.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>