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>
msm: kgsl: enable CPU IRQ while suspending context

Make sure that CPU interrupt is enabled before calling adreno
suspend_context when moving from NAP/SLEEP to SLUMBER state.
We send commands to GPU in adreno_drawctxt_switch() which will generate
GPU interrupt. Since CPU interrupt is not enabled, adreno_idle() will
fail because pending interrupt will not be served by CPU this causes
false GPU hang.
msm: kgsl: Restrict GPU memory object alignment to 32MB

The code was being very generous in giving the user the ability to
align GPU objects on 4GB boundaries. Cut the restriction down to
32MB which is still pretty generous but within the realm of
possiblity (especially for a 32 bit GPU).  While we are at it fix
a bug in the page_alloc code that was accidently resetting the
alignment to the alloc page size.  The alignment cannot be _less_
than the max page_alloc size but there certainly isn't any reason
why you couldn't specify a larger alignment if you so wished.