gallium: Add pipe cap for primitive restart with fixed index

gallium: Add pipe cap for primitive restart with fixed index

Adds PIPE_CAP_PRIMITIVE_RESTART_FIXED_INDEX which is a subset of the
primitive restart cap for when the hardware can only support the fixed
indices specified in GLES.

The switch statements were automatically modified with this command:

find \( \( -name \*.cpp -o -name \*.c \) \! -type l \) \
 -exec sed -i -r \
 's/^(\s*case\s+PIPE_CAP_PRIMITIVE_RESTART)\s*:.*$/\0\n\1_FIXED_INDEX:/' \
 {} \;

v2: Add a note in screen.rst

Reviewed-by: Eric Anholt <eric@anholt.net> (v1)
Reviewed by: Erik Faye-Lund <erik.faye-lund@collabora.com>

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5559>