drm/asahi: build VmReadGuard::covers_range only with the G15 backend

Only the G15 command checks use it. With DRM_ASAHI_M3_T8122=y and
DRM_ASAHI_G15=n, W=1 warns that it is never used.

Signed-off-by: Eryk Wieliczko <eryk@wieliczko.ninja>
Assisted-by: Claude:claude-opus-5-5

Author: erykwieliczko

drivers/gpu/drm/asahi/mmu.rs

@@ -363,6 +363,7 @@ pub(crate) struct VmReadGuard(VmExecGuard);
 
 #[cfg(CONFIG_DRM_ASAHI_UAT_ADOPTION)]
 impl VmReadGuard {
+    #[cfg(CONFIG_DRM_ASAHI_G15)]
     pub(crate) fn covers_range(&mut self, address: u64, size: u64, read: bool, write: bool) -> bool {
         Vm::covers_range_locked(&mut self.0, address, size, read, write)
     }