drm/i915: don't wait for power cycle when waiting for power off

Function ironlake_wait_panel_off should just wait for the power off
delay, while function ironlake_wait_panel_power_cycle should wait for
the panel cycle (that's required after we turn the panel off, before
we enable it again).

The problem is that, currently, ironlake_wait_panel_off is waiting not
just for the panel to be off, but also for the power cycle delay and
the backlight off delay. This function relies on the PP_STATUS bits
3:0, which are not documented and not supposed to be used. A quick
analysis of the values we get while waiting quickly shows that power
off is reached while bits 3:0 are still 0x1, and the time it takes to
become 0x0 is the power cycle delay.

On my system with backlight off delay of 200ms, power down delay of
50ms and power cycle delay of 500ms, this is what I get:
 - Start waiting with value 0x80000008, timestamp 6.429364.
 - Jumps to 0xa0000003, timestamp 6.431360 (time waited: 0.001996)
 - Jumps to 0xa0000002, timestamp 6.631277 (time waited: 0.201913)
 - Jumps to 0x08000001, timestamp 6.681258 (time waited: 0.251894)
 - Jumps to 0x00000000, timestamp 7.192012 (time waited: 0.762648)

As you can see, ironlake_wait_panel_off is sleeping 760ms instead of
the expected 50ms: the first 200ms matches the backlight off delay
(which we should already have waited for!), then the 50ms for the real
panel off delay, then the 500ms for the panel power cycle.

This patch makes is look just at bits 31 and 29:28, which will ignore
the panel power cycle.

And just to be clear: this saves 500ms on my system every time we
disable the panel. But we can still save 200ms more (the backlight off
delay) on the next patches.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuougseek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>