nv50_2d: Document two previously unknown methods
Provides missing documentation of two 2D methods from nouveau's rnndb:
NV*_2D_SET_PIXELS_FROM_MEMORY_SAFE_OVERLAP
NV*_2D_SET_PIXELS_FROM_MEMORY_CORRAL_SIZE
Provides missing documentation of two 2D methods from nouveau's rnndb:
NV*_2D_SET_PIXELS_FROM_MEMORY_SAFE_OVERLAP
NV*_2D_SET_PIXELS_FROM_MEMORY_CORRAL_SIZE