gfxstream-guest: update offset to correct value

In CoherentMemory::subAllocate function, we should
also update offset to the correct values.

Test: boot with skiavk enabled and the textures
should not be corrupted

Reviewed-by: Marcin Radomski <dextero@google.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/33018>

src/gfxstream/guest/vulkan_enc/HostVisibleMemoryVirtualization.cpp

@@ -43,6 +43,7 @@ bool CoherentMemory::subAllocate(uint64_t size, uint8_t** ptr, uint64_t& offset)
     if (!block) return false;
 
     *ptr = mBaseAddr + block->ofs;
+    offset = block->ofs;
     return true;
 }