Frontbuffer rendering and page flips can race with each other
and this can potentialy cause issues with PSR2 selective fetch.

And because pipe/crtc updates are time sentive we can't grab the
PSR lock after intel_pipe_update_start() and before
intel_pipe_update_end().

So here adding the lock and unlock functions and calls, the
proper PSR2 selective fetch handling will come in a separated patch.

v2:
- fixed new functions documentation
Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Cc: Jouni Högander <jouni.hogander@intel.com>
Cc: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220405155344.47219-2-jose.souza@intel.com