vga_switcheroo: Add missing locking

The following functions iterate over the client list, invoke client
callbacks or invoke handler callbacks without locking anything at all:

- Introduced by c8e9cf7b ("vga_switcheroo: Add a helper function to
  get the client state"):
  vga_switcheroo_get_client_state()

- Introduced by 0d69704a ("gpu/vga_switcheroo: add driver control
  power feature. (v3)"):
  vga_switcheroo_set_dynamic_switch()
  vga_switcheroo_runtime_suspend()
  vga_switcheroo_runtime_resume()
  vga_switcheroo_runtime_resume_hdmi_audio()

Refactor vga_switcheroo_runtime_resume_hdmi_audio() a bit to be able to
release vgasr_mutex immediately after iterating over the client list.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>