Fix display underruns on Pineview with 2048x1280 VGA display.

Fix display underruns on Pineview with 2048x1280 VGA display.

Higher dot clocks were working because pixel doubling was enabled.

Lower the apparently arbitrary threshold so it's enabled for 2048x1280.

Intel hasn't felt moved to document any of this, so this is purely empirical.

The original threshold was introduced here:
http://cgit.freedesktop.org/~anholt/xf86-video-intel/commit/?id=8fcf9a81179ee8577ddab5e904c58fbfd14cf59c

BUG=chromium:219562
TEST=All previously working U3011 modes + 2048x1280 work
Change-Id: I69f47b6ef292d65dc1a3ff6288c33eaa4c1cd894
Reviewed-on: https://gerrit.chromium.org/gerrit/49434
Reviewed-by: Stuart Abercrombie <sabercrombie@chromium.org>
Tested-by: Stuart Abercrombie <sabercrombie@chromium.org>
Commit-Queue: Stuart Abercrombie <sabercrombie@chromium.org>