glsl: Fix location bias for patch variables.

glsl: Fix location bias for patch variables.

We need to subtract VARYING_SLOT_PATCH0, not VARYING_SLOT_VAR0.

Since "patch" only applies to inputs and outputs, we can just handle
this once outside the switch statement, rather than replicating the
check twice and complicating the earlier conditions.

Cc: mesa-stable@lists.freedesktop.org
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>
(cherry picked from commit 398428f40684addd0a1465cf268436e8b1865351)