mesa: Add a helper function for determining the restart index.

The derived state approach currently used (_RestartIndex) doesn't work:
in the GL_PRIMITIVE_RESTART_FIXED_INDEX case, the restart index depends
on the index buffer's data type, and that isn't known until draw time.

The existing code also fails to obey the GL 4.3 rules which say that
FIXED_INDEX takes precedence over normal primitive restart.

This helper function correctly determines the restart index, and will
replace the derived state.

NOTE: This is a candidate for the 9.1 branch.

Signed-off-by: Kenneth Graunke <[email protected]>
Reviewed-by: Eric Anholt <[email protected]>
Reviewed-by: Ian Romanick <[email protected]>

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