AArch64: Correction of struct surface_t

The struct in the union with GGLSurface in struct surface_t assumed
that the reserved field, which corresponds to the GGLsizei version
field in the GGLSurface structure (where GGLsizei is define as being
ssize_t in the system/core/include/pixelflinger/pixelflinger.h header
file) is uint32_t. That is appropriate to the 32-bit system but is
not appropriate to the 64-bit system, where ssize_t is 64-bits rather
than 32-bits.

The 32-bit system has also been tested and has no regression
because of this change.

Change-Id: I5cd84fd4a18fa0e63e27975f7b3d3e95b99cea58
Signed-off-by: Marcus Oakland <marcus.oakland@arm.com>