drm/amd/amdgpu/psp_v11_0: Make local function 'psp_v11_0_wait_for_bootloader()' static

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/amd/amdgpu/psp_v11_0.c:223:5: warning: no previous prototype for ‘psp_v11_0_wait_for_bootloader’ [-Wmissing-prototypes]

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Hawking Zhang <Hawking.Zhang@amd.com>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>