drm/i915: disable IPS while getting the pipe CRCs.

For some yet-undiscovered reason, when IPS gets enabled, the pipe CRC
changes. Since hsw_enable_ips() doesn't really guarantees to enable
IPS (it depends on package C-states), we can't really predict if IPS
is enabled or disabled while running our CRC tests, so let's just
completely disable IPS while pipe CRCs are being used.

If we find a way to make IPS not change the pipe CRC result, we may
want to fix IPS and then revert this patch. While this doesn't happen,
let's merge this patch, so every IGT test relying on the CRCs can
work on pipe A.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=72864


Testcase: igt/kms_cursor_crc (and others)
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>