CHROMIUM: drm/exynos: Fix compile warning in dp_core

CHROMIUM: drm/exynos: Fix compile warning in dp_core

In function 'exynos_dp_hotplug':
warning: 'voltage_swing' may be used uninitialized in this function
[-Wmaybe-uninitialized]

While it may be impossible for dp->link_train.lane_count to be 0, the
compiler can't know that.  If it is 0 for some reason, then voltage_swing
does not get set in the lane loop and therefore
exynos_dp_check_max_cr_loop() gets called with a bogus voltage_swing.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
BUG=chromium-os:36770
TEST=builds without the warning

Change-Id: Ia19a0439487995bd600570e0f32bf4a4c31e4407
Reviewed-on: https://gerrit.chromium.org/gerrit/38901
Commit-Ready: Daniel Kurtz <djkurtz@chromium.org>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Tested-by: Daniel Kurtz <djkurtz@chromium.org>