pvr: Handle special built-in variable loading in vertex shader

pvr: Handle special built-in variable loading in vertex shader

For now always allocate space for the special built-in variables
that need filling in for the vertex shader. Ideally we should get
the compiler to analyze the shader and tell the driver if they are
used so that we don't reserve and fill them needlessly.

Signed-off-by: Karmjit Mahil <Karmjit.Mahil@imgtec.com>
Reviewed-by: Frank Binns <frank.binns@imgtec.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21590>