panfrost: Introduce PAN_MAX_VARYINGS define

panfrost: Introduce PAN_MAX_VARYINGS define

The number of varying records we need to reserve in the worst case is
greater than the number of source-level varyings we advertise
(gl_Position, gl_PointSize...)

We advertise MAX_VARYINGS source level varyings, which means anywhere we
manipulate varyings we need up to (MAX_VARYINGS + max non-source level
varyings) records. Add a PAN_MAX_VARYINGS define for this and use it
throughout.

Fixes a buffer overflow in Piglit glsl-max-varyings, which now passes
instead of crashes.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13070>