[R300 and other radeons] MergedFB lockups

[Vladimir Dergachev]

I think I found the cause of lockups in VB mode - they were due to cursor 
updating function in radeon_mergedfb.c calling OUTREGP() which in turn 
called INREG.

When silken mouse is enabled this function could be called at any time 
which would do *bad* things when CP engine is active.

The fix of putting RADEONWaitForIdleMMIO() works fine on my setup.

I have *no* idea why this worked with immediate mode at all and why no 
issues were reported by R200 and Radeon users (well, I only looked through 
the mailing lists, perhaps there is something on bugzilla but I don't know 
how to use that efficiently)

Also, I have no idea why the code in radeon_cursor.c that writes images 
directly into framebuffer memory works - according to the manual any 
writes into framebuffer while GUI is active should cause a hard lock.

However, I could not produce any lockups with it, and so left it as is.

                              best

                                 Vladimir Dergachev