freedreno/a6xx: don't set SP_FS_CTRL_REG0.VARYING for fragcoord
Similar change to 5785bcc8. It appears on a6xx and in fact this could cause varying corruption before the FS had a chance to consume the varyings from varying storage.
Closes: #2838 (closed) Signed-off-by: Rob Clark robdclark@chromium.org