surfaceflinger: fix -Wsign-compare warnings

warning: comparison of integers of different signs: 'int' and 'size_t'
(aka 'unsigned int') [-Wsign-compare]

arning: comparison of integers of different signs: 'int32_t' (aka
'int') and 'const uint32_t' (aka 'const unsigned int')
[-Wsign-compare]

Change-Id: I823257aa7218c5fd492a3277853210db539bb2e2