udlfb: don't switch if we are switching to the same videomode

The udlfb driver reprograms the hardware everytime the user switches the
console, that makes quite unusable when working on the console.

This patch makes the driver remember the videomode we are in and avoid
reprogramming the hardware if we switch to the same videomode.

We mask the "activate" field and the "FB_VMODE_SMOOTH_XPAN" flag when
comparing the videomode, because they cause spurious switches when
switching to and from the Xserver.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>