drm/ttm: fix compilation on ARCH=um

Even if it's probably not really useful, it can get selected
by e.g. randconfig builds, and then failing to compile is an
annoyance. Unfortunately, it's hard to fix in Kconfig, since
DRM_TTM is selected by many things that don't really depend
on any specific architecture, and just depend on PCI (which
is indeed now available in ARCH=um via simulation/emulation).

Fix this in the code instead by just ifdef'ing the relevant
two lines that depend on "real X86".

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20211220111519.a4c8c6eff702.Ie4cf4e68698f6a9f546b83379bc52c266504424f@changeid