surfaceflinger: remove hwcId < 0 path in doComposeSurfaces

Remove the check for hwcId and re-indent the code.  hwcId is never
less than zero.  Even if it did,
getCompositionType(DISPLAY_ID_INVALID) would return
HWC2::Composition::Client and it would still work.

The motivation is that the hwcId < 0 path has gone outdated since
the introduction of legacy sRGB saturation matrix.

Bug: 78891890
Test: virtual displays
Change-Id: I6f3e779bc0f8ef21d33ce7de49ddbfa95757e473