drm/i915: increase the SWSCI DSLP default timeout to 50ms

The spec says the default timeout should be 2ms, but on my machine
this doesn't seem to be enough. Sometimes it works, sometimes I get
these messages when booting:
  - SWSCI request timed out
  - SWSCI request already in progress

And my guess is that the "already in progress" message is because the
first one is still happening.

I did some experiments on my machine (that has CONFIG_HZ=1000) and the
wait_for function usually takes 4-6 jiffies to finish, but I've seen
up to 9. So increase the timeout to 50ms. We only expect to wait for
the actual amount of time the operation takes, so even a huge timeout
shouldn't delay us more than what the hardware actually requires.

Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_opregion.c

@@ -258,7 +258,9 @@ static int swsci(struct drm_device *dev, u32 function, u32 parm, u32 *parm_out)
 	/* Driver sleep timeout in ms. */
 	dslp = ioread32(&swsci->dslp);
 	if (!dslp) {
-		dslp = 2;
+		/* The spec says 2ms should be the default, but it's too small
+		 * for some machines. */
+		dslp = 50;
 	} else if (dslp > 500) {
 		/* Hey bios, trust must be earned. */
 		WARN_ONCE(1, "excessive driver sleep timeout (DSPL) %u\n", dslp);