drm/i915: Fix panel fitting regression since 734b4157

The crtc mode fixup is run after the encoders adjust the mode to fit on
their output, so don't reset the mode!

Fixes:

  Bug 29057 - display corruption under 800x600 on netbook
              (1024x600) with 'Full Aspect' scaling
  https://bugs.freedesktop.org/show_bug.cgi?id=29057

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Tested-by: Xun Fang <xunx.fang@intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>