drm/msm: Allow a5xx to mark the RPTR shadow as privileged

Newer microcode versions have support for the CP_WHERE_AM_I opcode which
allows the RPTR shadow memory to be marked as privileged to protect it
from corruption. Move the RPTR shadow into its own buffer and protect it
it if the current microcode version supports the new feature.

We can also re-enable preemption for those targets that support
CP_WHERE_AM_I. Start out by preemptively assuming that we can enable
preemption and disable it in a5xx_hw_init if the microcode version comes
back as too old.

Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>