drm/amd/display: Export symbols for KUnit test modules

drm/amd/display: Export symbols for KUnit test modules

Add missing EXPORT_IF_KUNIT() calls for amdgpu_dm_psr_set_event,
amdgpu_dm_ism_init, and amdgpu_dm_ism_fini so that the KUnit test
modules can resolve these symbols when built as modules, i.e.,
CONFIG_DRM_AMD_DC_KUNIT_TEST=m.

Fixes: 34f281489976 ("drm/amd/display: Add KUnit tests for amdgpu_dm_psr_set_event")
Fixes: a3142b13fe9f ("drm/amd/display: Add more KUnit tests for amdgpu_dm_ism")
Assisted-by: Copilot:Claude-Opus-4.6
Reviewed-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>