st/mesa: don't allocate bitmap drawing state until needed

st/mesa: don't allocate bitmap drawing state until needed

Most apps don't use glBitmap so don't allocate the bitmap cache or
gallium state objects/shaders/etc until the first call to st_Bitmap().

v2: simplify a conditional, per Gustaw Smolarczyk.

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>