Remove leftover __DRI{screen,drawable,context}Private references

Remove leftover __DRI{screen,drawable,context}Private references

As part of the DRI driver interface rewrite I merged __DRIscreenPrivate
and __DRIscreen, and likewise for __DRIdrawablePrivate and
__DRIcontextPrivate.  I left typedefs in place though, to avoid renaming
all the *Private use internal to the driver.  That was probably a
mistake, and it turns out a one-line find+sed combo can do the mass
rename.  Better late than never.