drm/msm/sde: add hw log mask for REGDMA

Setup log_mask for the REGDMA hardware block.  This is used to
enable register write logs via the "hw_log_mask" debugfs node.

Change-Id: Ifff9512c4354677bc924279031609712469d5f96
Signed-off-by: Steve Cohen <cohens@codeaurora.org>