drm/i915: Don't rmw PIPESTAT enable bits

drm/i915: Don't rmw PIPESTAT enable bits

i830 seems to occasionally forget the PIPESTAT enable bits when
we read the register. These aren't the only registers on i830 that
have problems with RMW, as reading the double buffered plane
registers returns the latched value rather than the last written
value. So something similar is perhaps going on with PIPESTAT.

This corruption results on vblank interrupts occasionally turning off
on their own, which leads to vblank timeouts and generally a stuck
display subsystem.

So let's not RMW the pipestat enable bits, and instead use the cached
copy we have around.

Cc: Imre Deak <imre.deak@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170914151731.5034-1-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>