msm: kgsl: Reorganize draw context validation

Verify the incoming draw context flags are correct *before* bothering
to initalize the memory. This saves us from having to jump through
hoops to take it down later.

Change-Id: Ic0dedbada00fc63752d2763ec18e1bfc3bb6410f
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>