drm/msm: fix issue dumping sde registers

Fix an issue where we try to print out vbif_nrt area, which is
not supported on current targets. Check register length before
printing to avoid extraneous error print. Change va_args
checks to avoid incorrect failure to parse all dump areas print.

Change-Id: Icd057ed7aabb17d1211432a512ff80828f80f2c3
Signed-off-by: Lloyd Atkinson <latkinso@codeaurora.org>