drm: msm: Remove stddef.h include from uapi

Having this include here causes sparse to spew an error about
including a file outside of the kerenel repo. There's a
linux/stddef.h file that could be included instead, but things
compile without this include anyway so just remove it.

Change-Id: I320bc9aa23c4e8d75a689efb4c4d0dd2db5e150b
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>