drm/amd/display: make is_synaptics_cascaded_panamera static

drm/amd/display: make is_synaptics_cascaded_panamera static

This symbol is not used outside of amdgpu_dm_mst_types.c, so marks it
static.

drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm_mst_types.c:211:6: warning: no previous prototype for ‘is_synaptics_cascaded_panamera’.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Link: https://bugzilla.openanolis.cn/show_bug.cgi?id=4548
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>