drm/gma500: remove unused declarations in psb_intel_drv.h

drm/gma500: remove unused declarations in psb_intel_drv.h

psb_intel_sdvo_supports_hotplug(), psb_intel_sdvo_set_hotplug()
and psb_intel_sdvo_find() have been removed since
commit 871c60156dbe ("drm/gma500: Remove dead code"),
so remove them.

Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220913024847.552254-5-cuigaosheng1@huawei.com