drm/ttm: cleanup coding style in ttm_bo_api.h

Extern is the default for function declerations anyway and this
solves a bunch of 80char per line issues.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>