msm: kgsl: Only access and map the shadermem for a3xx snapshot

The "kgsl_3d0_shader_mem" resource is only used for a3xx snapshot, yet
we check for it and print a message on every single target. Luckily we
were already mapping the object on demand in the snapshot code so it
is easy enough to move the resource check there as well (and use one of
the fancy new devm_ functions to boot).

While we are kicking around in there, we don't need a special regread
function for shadermem because memcpy_fromio does the same task in
a better way. Removing the regread gives us the chance to tighten
the adreno_regread code to our benefit as well.

Change-Id: Ic0dedbadd93d4f96528e9e75a1325c37a5b0ea90
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>