asahi: Fix warning building for macOS 12.0

Unnecessary rename that breaks forward compatibility... but Apple says
this is just NULL. Do the simpler thing.

../src/asahi/lib/agx_device.c:290:35: warning: 'kIOMasterPortDefault' is deprecated: first deprecated in macOS 12.0 [-Wdeprecated-declarations]
      IOServiceGetMatchingService(kIOMasterPortDefault, matching);
                                  ^~~~~~~~~~~~~~~~~~~~
                                  kIOMainPortDefault

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
86 jobs for !18121 with asahi/warn-fix in 9 seconds (queued for 9 seconds)
merge request