st/mesa: be more careful about state validation in st_Bitmap()

st/mesa: be more careful about state validation in st_Bitmap()

If the only dirty state is mesa's _NEW_PROGRAM_CONSTANTS flag, we can
skip state validation before drawing a bitmap since that state doesn't
effect bitmap rendering.

This further increases the performance of the ipers demo on llvmpipe
to about what it was before commit 36c93a6fae27561.

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