gallium: add PIPE_SHADER_CAP_MAX_UNROLL_ITERATIONS_HINT

gallium: add PIPE_SHADER_CAP_MAX_UNROLL_ITERATIONS_HINT

This avoids a serious r600g bug leading to a GPU hang.
The chances this bug will get fixed are pretty low now.

I deeply regret listening to others and not pushing this patch, leaving
other users with a GPU-crashing driver. Yes, it should be fixed
in the compiler and it's ugly, but users couldn't care less about that.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=86720

Cc: 11.0 10.6 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Brian Paul <brianp@vmware.com>