drm: Replace kzalloc with kcalloc

Prefer kcalloc over kzalloc to allocate an array.
This patch fixes checkcpatch issue.

Signed-off-by: Harsha Sharma <harshasharmaiitr@gmail.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20171013073747.29877-1-harshasharmaiitr@gmail.com
Link: https://patchwork.freedesktop.org/patch/msgid/20171013073747.29877-1-harshasharmaiitr@gmail.com