st/mesa: Limit GL_MAX_[NATIVE_]PROGRAM_PARAMETERS_ARB to 2048

st/mesa: Limit GL_MAX_[NATIVE_]PROGRAM_PARAMETERS_ARB to 2048

Piglit's vp-max-array test creates a vertex program containing a uniform
array sized to the value of GL_MAX_NATIVE_PROGRAM_PARAMETERS_ARB.  Mesa
will then add additional state-var parameters for things like the MVP
matrix.

radeonsi currently exposes a value of 4096, derived from constant buffer
upload size.  This means the array will have 4096 elements, and the
extra MVP state-vars would get a prog_src_register::Index of over 4096.

Unfortunately, prog_src_register::Index is a signed 13-bit integer, so
values beyond 4096 end up turning into negative numbers.  Negative
source indexes are only valid for relative addressing, so this ends up
generating illegal IR.

In prog_to_nir, this would cause an out of bounds array access.
st_mesa_to_tgsi checks for a negative value, assumes it's bogus,
and remaps it to parameter 0 in order to get something in-range.
This isn't right - instead of reading the MVP matrix, it would read
the first element of the vertex program's large array.  But the test
only checks that the program compiles, so we never noticed that it
was broken.

This patch limits the size of the program limits, with the understanding
that we may need to generate additional state-vars internally.  i965 has
exposed 1024 for this limit for years, so I don't expect lowering it to
2048 will cause any practical problems for radeonsi or other drivers.

Fixes vp-max-array with prog_to_nir.c.

Cc: "19.0" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Eric Anholt <eric@anholt.net>