drm/msm: fix issue dumping sde registers

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>
Signed-off-by: Abhinav Kumar <abhinavk@codeaurora.org>