kgsl: do not vmap/memset to zero-out pages

b/18402205 External reports: Video playback failing on Flo after upgrade to
	   Lollipop

Change-Id: I358328ba2bd543d77e4218f32b0695c2f6f6e6c9
Signed-off-by: Iliyan Malchev <malchev@google.com>