msm: kgsl: hold a reference on pages found with memdesc_sg_virt()

The GPU could write to pages gathered with this function, so
references should be held while they are in use.
Since the function was basically a broken version of
get_user_pages(), just call that instead.

Change-Id: I5cca19e06bc94eff12d0c6e937576fd303c10ab8
Signed-off-by: Jeremy Gebben <jgebben@codeaurora.org>