nir: Fix decomposed_prmcnt copy-paste error
What does this MR do and why?
nir: Fix decomposed_prmcnt copy-paste error
Fix defect reported by Coverity Scan.
Copy-paste error (COPY_PASTE_ERROR)
copy_paste_error: prmcnt in prmcnt = -1 looks like a copy-paste error.
Fixes: cc3f20ca6c1 ("nir: Also gather decomposed primitive count")
Signed-off-by: Vinson Lee <vlee@freedesktop.org>