FTL: Silence warnings for FTL_FAKE_GUARD

Do not return an lvalue reference to avoid warnings like:

    warning: returning variable 'mPhysicalDisplays' by reference
    requires holding mutex 'mStateLock' exclusively
    [-Wthread-safety-reference-return]

The removal of the lambda means that FTL_FAKE_GUARD(m, e) can no longer
be used in a function that REQUIRES(m), which was redundant. Remove one
such instance in SF.

Bug: 354740314
Bug: 185536303
Flag: EXEMPT compile-time change
Test: m surfaceflinger
Merged-In: I35b029885e9c541c1715c280d4f3542de5096145
Change-Id: I35b029885e9c541c1715c280d4f3542de5096145