drm/radeon/dpm: fix compilation with certain versions of gcc

Add #include <linux/seq_file.h> to *_dpm.c files

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

diff --cc drivers/gpu/drm/radeon/ni_dpm.c
Simple merge

diff --cc drivers/gpu/drm/radeon/rv6xx_dpm.c
Simple merge

diff --cc drivers/gpu/drm/radeon/rv770_dpm.c
Simple merge

diff --cc drivers/gpu/drm/radeon/si_dpm.c
Simple merge

diff --cc drivers/gpu/drm/radeon/sumo_dpm.c
Simple merge

diff --cc drivers/gpu/drm/radeon/trinity_dpm.c
Simple merge