video_core/memory_manager: Make FindFreeRegion() a const member function

This doesn't modify internal state, so it can be made a const member
function.

src/video_core/memory_manager.cpp

@@ -77,9 +77,10 @@ GPUVAddr MemoryManager::UnmapBuffer(GPUVAddr gpu_addr, u64 size) {
     return gpu_addr;
 }
 
-GPUVAddr MemoryManager::FindFreeRegion(GPUVAddr region_start, u64 size) {
+GPUVAddr MemoryManager::FindFreeRegion(GPUVAddr region_start, u64 size) const {
     // Find the first Free VMA.
-    const VMAHandle vma_handle{std::find_if(vma_map.begin(), vma_map.end(), [&](const auto& vma) {
+    const VMAHandle vma_handle{
+        std::find_if(vma_map.begin(), vma_map.end(), [region_start, size](const auto& vma) {
             if (vma.second.type != VirtualMemoryArea::Type::Unmapped) {
                 return false;
             }

src/video_core/memory_manager.h

@@ -127,7 +127,7 @@ private:
     void UpdatePageTableForVMA(const VirtualMemoryArea& vma);
 
     /// Finds a free (unmapped region) of the specified size starting at the specified address.
-    GPUVAddr FindFreeRegion(GPUVAddr region_start, u64 size);
+    GPUVAddr FindFreeRegion(GPUVAddr region_start, u64 size) const;
 
 private:
     static constexpr u64 page_bits{16};