drm/radeon/radeon_kms: Include header containing our own prototypes

drm/radeon/radeon_kms: Include header containing our own prototypes

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

 drivers/gpu/drm/radeon/radeon_kms.c:61:6: warning: no previous prototype for ‘radeon_driver_unload_kms’ [-Wmissing-prototypes]
 drivers/gpu/drm/radeon/radeon_kms.c:104:5: warning: no previous prototype for ‘radeon_driver_load_kms’ [-Wmissing-prototypes]
 drivers/gpu/drm/radeon/radeon_kms.c:618:6: warning: no previous prototype for ‘radeon_driver_lastclose_kms’ [-Wmissing-prototypes]
 drivers/gpu/drm/radeon/radeon_kms.c:633:5: warning: no previous prototype for ‘radeon_driver_open_kms’ [-Wmissing-prototypes]
 drivers/gpu/drm/radeon/radeon_kms.c:704:6: warning: no previous prototype for ‘radeon_driver_postclose_kms’ [-Wmissing-prototypes]
 drivers/gpu/drm/radeon/radeon_kms.c:755:5: warning: no previous prototype for ‘radeon_get_vblank_counter_kms’ [-Wmissing-prototypes]
 drivers/gpu/drm/radeon/radeon_kms.c:825:5: warning: no previous prototype for ‘radeon_enable_vblank_kms’ [-Wmissing-prototypes]
 drivers/gpu/drm/radeon/radeon_kms.c:852:6: warning: no previous prototype for ‘radeon_disable_vblank_kms’ [-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: 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>