Add some new functions to manipulate GPU registers.  gpu_read64 and
gpu_write64 can read/write a 64 bit value to two 32 bit registers.
For 4XX and older these are normally perfcounter registers, but
future targets will use 64 bit addressing so there will be many
more spots where a 64 bit read and write are needed.

gpu_rmw() does a read/modify/write on a 32 bit register given a mask
and bits to OR in.
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>