fbdev: uvesafb: don't build on UML

fbdev: uvesafb: don't build on UML

The uvesafb fbdev driver uses memory management information that is not
available on ARCH=um, so don't allow this driver to be built on UML.

Prevents these build errors:

../drivers/video/fbdev/uvesafb.c: In function ‘uvesafb_vbe_init’:
../drivers/video/fbdev/uvesafb.c:807:21: error: ‘__supported_pte_mask’ undeclared (first use in this function)
  807 |                 if (__supported_pte_mask & _PAGE_NX) {
../drivers/video/fbdev/uvesafb.c:807:44: error: ‘_PAGE_NX’ undeclared (first use in this function)
  807 |                 if (__supported_pte_mask & _PAGE_NX) {

Fixes: 68f5d3f3b654 ("um: add PCI over virtio emulation driver")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Johannes Berg <johannes@sipsolutions.net>
Cc: Richard Weinberger <richard@nod.at>
Cc: linux-um@lists.infradead.org
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Helge Deller <deller@gmx.de>
Cc: linux-fbdev@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Cc: Michal Januszewski <spock@gentoo.org>
Signed-off-by: Helge Deller <deller@gmx.de>