panfrost: Dynamically allocate shader variants

panfrost: Dynamically allocate shader variants

This fixes a crash in LZDoom where over 16 shader variants are needed
for a few shaders in some maps, and should also save a few kilobytes
of RAM as most of the time only one or two variants of the 8 previously
allocated are actually needed.

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>