drm/i915: vlv: don't unmask IIR[DISPLAY_PIPE_A/B_VBLANK] interrupt

drm/i915: vlv: don't unmask IIR[DISPLAY_PIPE_A/B_VBLANK] interrupt

Bspec and the code suggests that the interrupt signaled by IIR[7,5]
(DISPLAY_PIPE_A/B_VBLANK) is a first level IRQ flag for the second
level PIPEA/BSTAT[2] (Start of Vertical Blank) interrupt. Measuring
the relative timings of when IIR[7] and PIPEASTAT[1,2] get set and
checking the effect of unmasking different pipestat and IIR events
shows that this isn't so:

First, ISR/IIR[7] gets set independently of PIPEASTAT[18] (Start of
Vertical Blank Enable) or any other pipestat enable bit, so it isn't
a first level IRQ bit showing the state of PIPEASTAT[2], but is
connected directly to the timing generator.

Second, setting only PIPEASTAT[18] and leaving all other pipestat events
disabled, IIR[6] (DISPLAY_PIPE_A_EVENT) gets set close to the moment when
PIPEASTAT[2] gets set, so the former is a first level interrupt flag for
the latter. The bspec is rather unclear about this, but I also assume
that IIR[6] signals all pipestat A events, except PIPEASTAT[31] (FIFO
Under-run Status).

Third, IIR[7] is set close to the moment when PIPEASTAT[1] (Framestart
Interrupt) gets set, in the mode I used about 12usec after PIPEASTAT[2]
and IIR[6] gets set. This means the IIR[7] isn't marking the start of
vblank, but rather signals the framestart event.

Based on the above, we don't need to unmask IIR[7] when waiting for
start of vblank events, but we can rely on IIR[6] being always unmasked,
which will signal when PIPEASTAT[2] gets set. Doing this will also get
rid of the overhead of getting an interrupt and servicing IIR[7], which
is atm raised always some time after IIR[6]/PIPEASTAT[2] is raised.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>