input: touchscreen: remove data pointer from driver structure

Remove data pointer from the driver structure which is obtained
from user-space. Exclude it from driver structure and move it
outside as another global data pointer.
The definition of CMD_HEAD_LENGTH is meant to exclude the
last member (*data) from the copy from userspace. This change
is meant to keep the definition of CMD_HEAD_LENGTH simple,
and handle the data separately.

Change-Id: I62f14c744e5c331718b6d84f67a4e89b511c9f44
Signed-off-by: Shantanu Jain <shjain@codeaurora.org>