drm/i915: Wait up to 3ms for the pcu to ack the cdclk change request on SKL

drm/i915: Wait up to 3ms for the pcu to ack the cdclk change request on SKL

Bspec tells us to keep bashing the PCU for up to 3ms when trying to
inform it about an upcoming change in the cdclk frequency. Currently
we only keep at it for 15*10usec (+ whatever delays gets added by
the sandybridge_pcode_read() itself). Let's change the limit to 3ms.

I decided to keep 10 usec delay per iteration for now, even though
the spec doesn't really tell us to do that.

Cc: stable@vger.kernel.org
Fixes: f93a42813434 ("drm/i915/skl: Deinit/init the display at suspend/resume")
Cc: David Weinehall <david.weinehall@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1468416723-23440-1-git-send-email-ville.syrjala@linux.intel.com
Tested-by: David Weinehall <david.weinehall@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>