minui: Refactor GRSurfaceFbdev.

- Adds Create() that returns a GRSurfaceFbdev instance.
- Moves away from using the copy ctor (precisely assignment operator) of
  GRSurfaceFbdev.
- Moves the GRSurfaceFbdev deallocation code into GRSurfaceFbdev's dtor.
- Manages MinuiBackendFbdev::gr_framebuffer with std::unique_ptr.

Test: mmma -j bootable/recovery
Test: `Run graphics test` on taimen.
Change-Id: I8e67cda7bc3a2feec0790124d035caa36fb58a89