msm: mdss: Update wr mem fields with correct names

wr mem start and continue were incorrectly named in
the documentation and dtsi files. Changing the names
to wr-mem-start and wr-mem-continue to more accurately
reflect the fields.

Change-Id: If2cdd410fd50a4690dcb6c87cb0b9f4818848848
CRs-Fixed: 578028
Signed-off-by: Casey Piper <cpiper@codeaurora.org>