drivers/misc/sgi-xp: Adjust references in UV kernel modules

Remove the define is_uv() is_uv_system and just use the latter as is.
This removes a conflict with a new symbol in the generated uv_mmrs.h
file (is_uv()).

Signed-off-by: Mike Travis <mike.travis@hpe.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Dimitri Sivanich <dimitri.sivanich@hpe.com>
Reviewed-by: Steve Wahl <steve.wahl@hpe.com>
Link: https://lkml.kernel.org/r/20201005203929.148656-4-mike.travis@hpe.com