msm: mdss: Add plane_count range check in mdss WFD

For any given output buffer to the MDSS WFD, it is necessary to check
the range of the plane_count against the MAX_PLANES definition, in order
to avoid any out of bound access.

CRs-Fixed: 2028702
Change-Id: I4f1497a3a2e4ca2d30fc268e68cfdacc0d8539ea
Signed-off-by: Benjamin Chan <bkchan@codeaurora.org>