freedreno/a3xx: fix blend state corruption issue

freedreno/a3xx: fix blend state corruption issue

Using RMW on banked context registers is not safe.  The value read
could be the wrong one.  So if there has been a DRAW_IDX launched,
the RMW must be preceded by a WAIT_FOR_IDLE to ensure the read part
of RMW sees the correct value.

To avoid unnecessary WFI's, keep track if there is a need for WFI,
and only emit one if needed.  Furthermore, keep track if we even
need to update the register in the first place.

And to cut down on the amount of RMW to avoid excessive WFI's, at the
tiling/GMEM level we can always overwrite RB_RENDER_CONTROL, as the
state at beginning of draw/clear cmds (which we IB to) is always
undefined.  In the draw/clear commands, we always still use RMW (with
WFI if needed), but only if the register value actually changes.  (At
points where the current value cannot be known, the saved value is
reset to ~0, which includes bits outside of RBRC_DRAW_STATE, so there
never is chance for confusion.)

Signed-off-by: Rob Clark <robclark@freedesktop.org>