drm/exynos: add 1280x720 mode to exynos5 fimd

drm/exynos: add 1280x720 mode to exynos5 fimd

this patch adds 1280x720 mode to the fimd panel,
the lcd will switch to this mode when HDMI is plugged in
and will switch back to default mode when HDMI is plugged out.

BUG=chrome-os-partner:10851
TEST=Plugged in/out HDMI cable several times on snow,
     both in login screen and after login, the resolution
     switches accordingly

Change-Id: I4f02f158dbd9ae5aa1483e780dea00317aab9e1e
Signed-off-by: Shirish S <s.shirish@samsung.com>
Reviewed-on: https://gerrit.chromium.org/gerrit/26848
Reviewed-by: Mandeep Singh Baines <msb@chromium.org>
Reviewed-by: Sean Paul <seanpaul@chromium.org>