pmemalloc: fix GCC 4.7 errors

Signed-off-by: Michael Bestas <mikeioannina@gmail.com>

Change-Id: Id5d7bc86780bd716b97257400e15793c29ff4dfb
gralloc: fix gralloc module

Explicitly setting reserved (in C++) breaks future extensions of the
module format.  Removed these explicit settings for future
compatibility.

Change-Id: Ia7fa1e1ff73114a6eacb5b228e81ef1c30c32f43
Fix two trivial bugs found by gcc 4.7.

1) external.cpp: ext_commit.flags is uninitialized.
2) hwc_mdpcomp.cpp: signed int compared against another signed int
   which is unnecessarily casted to unsigned.

Bug: 7671979
Change-Id: I0e846cf0f7b161ce6f06f380af19a2864a3fd222