msm: kgsl: Add a function to map global buffers

Currently all global buffers are mapped when pagetables are created which
means that all global buffers need to be in place before the default
pagetable is created otherwise they will not be available in that context.
Add a new MMU function to map each global buffer. If the default pagetable
has been created then the new buffer can be mapped immediately, otherwise
all the buffers on the list will be mapped when the pagetable finally comes
along. Per-process pagetables will map everything from the list at create
time as they do now.

As a caveat this function only applies to the default pagetable. We will
not go back and retroactively map buffers into existing per-process
pagetables. Previously if somebody mapped a global buffer late it
wouldn't be available in the default pagetable either so it was easier to
debug. Now that this function will always successfully map buffers in the
default pagetable "late" buffers that got missed in a per-process table
will be harder to debug. In recognition of this the new map function will
trigger a warning if a buffer is mapped after the first per-process
pagetable is created. It won't stop breakage from happening but at least
it will be more easily debuggable.

Change-Id: Ic0dedbad2ed20769ea96bfdac3b707a30c6f9737
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>