drm: Use vsnprintf extension %ph

Using the extension saves a bit of code.

Miscellanea:

o Neaten and simplify dump_dp_payload_table
o Removed trailing blank space from output

$ size drivers/gpu/drm/drm_dp_mst_topology.o.* drivers/gpu/drm/tinydrm/*.o*
   text	   data	    bss	    dec	    hex	filename
  25848	      0	     16	  25864	   6508	drivers/gpu/drm/drm_dp_mst_topology.o.new
  26091	      0	     16	  26107	   65fb	drivers/gpu/drm/drm_dp_mst_topology.o.old
   3362	      2	      0	   3364	    d24	drivers/gpu/drm/tinydrm/mipi-dbi.o.new
   3376	      2	      0	   3378	    d32	drivers/gpu/drm/tinydrm/mipi-dbi.o.old

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/a78a21b5f34947da65473a0b7326922cda51a3be.1496187315.git.joe@perches.com