msm: kgsl: clean up use of memdesc->hostptr

Some external memory types were using hostptr to store
a userspace virtual address, but other code assumes it is a
kernel virtual address. Make memdesc->hostpr always
be the kernel virtual address and add a useraddr
field for the userspace virtual address.

Change-Id: Id4580a2ff34aeb15f2c1b26a7134f0fd4ec52a6e
Signed-off-by: Jeremy Gebben <jgebben@codeaurora.org>