turnip: don't set SP_FS_CTRL_REG0_VARYING if only fragcoord is used

turnip: don't set SP_FS_CTRL_REG0_VARYING if only fragcoord is used

Fixes artifacts in the subpasses demo, which has a shader using fragcoord
without any varyings. It looks like setting this bit when there are no
varyings can cause weirdness in some cases (without this change, if the
previous shader had <= 8 varyings it would work, but with 9 varyings it
would have artifacts).

Signed-off-by: Jonathan Marek <jonathan@marek.ca>
Reviewed-by: Eric Anholt <eric@anholt.net>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3143>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3143>