msm: kgsl: Add target specific probe functions

Add support for target specific probe functions to give individual
targets the flexibility to define their own device structs and do
other target specific setup that doesn't belong in the generic init.
To start all targets use a common helper probe function but that
is expected to change soon.

Change-Id: Ic0dedbad9e159be724368c7d2a496af151226cc0
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>