drm/msm/a6xx: Add zap shader load

The a6xx GPU powers on in secure mode which restricts what memory it can
write to. To get out of secure mode the GPU driver can write to
REG_A6XX_RBBM_SECVID_TRUST_CNTL but on targets that are "secure" that
register region is blocked and writes will cause the system to go down.

For those targets we need to execute a special sequence that involves
loadinga special shader that clears the GPU registers and use a PM4
sequence to pull the GPU out of secure. Add support for loading the zap
shader and executing the secure sequence. For targets that do not support
SCM or the specific SCM sequence this should fail and we would fall back
to writing the register.

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