mesa: fix save_ShadeModel() logic and add new comments

mesa: fix save_ShadeModel() logic and add new comments

This removes the test for _mesa_inside_dlist_begin_end().
If ctx->Driver.CurrentSavePrimitive==PRIM_UNKNOWN (the initial value),
_mesa_inside_dlist_begin_end() will, confusingly, return TRUE.
So we didn't set the ctx->ListState.Current.ShadeModel value and it
remained in its indeterminate state.

This didn't effect correctness, but it defeated the intended optimization
of dropping redundant glShadeModel() state changes in order to
coalesce sequences of drawing commands.

Verified with new piglit gl-1.0-dlist-shademodel test.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>