HWC2: Skip validate/present if display is off

Adds checks to ensure that the display is still on before attempting
to call into prepareFrame/commit (which in turn call into
validate/present).

Test: Cherry-pick from internal branch
Bug: 28596387
Change-Id: Iad87f02c3e44e86db43a15c258ee66b0cde14faa
(cherry picked from commit 00ac359a3b69670824ceab8d1dd5a1d5ad0ac5bd)
1 file changed