iris: Fix SO stride units for DrawTransformFeedback

iris: Fix SO stride units for DrawTransformFeedback

Mesa measures in DWords.  The hardware also claims to measure in DWords.
Except the SO_WRITE_OFFSET field is actually bits 31:2, with 1:0 MBZ.
Which means that it really measures in bytes.  So, convert to bytes.

Without this, our offset / stride denominator was 1/4th the size it
should be, leading to 4x the vertex count that we should have had.

Fixes GTF-GL46.gtf40.GL3Tests.transform_feedback2.transform_feedback2_two_buffers