drm/amd: Add HDP flush during jpeg init

During jpeg init, CPU writes to frame buffer which can be cached by HDP,
occasionally causing invalid header to be sent to MMSCH.  Perform HDP flush
after writing to frame buffer before continuing with jpeg init sequence.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Timmy Tsai <timmtsai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>