gralloc: Fix retrieving metadata state

The call to function GetMetaDataByReference was inadvertently removed
in commit 07f2c74, because `result` was an unused variable.
Restore that call, but do it properly avoiding the compilation issue.

This fixes the logspam:

  E qtigralloc: Unable to get metadata state

Change-Id: I2c2ee63df25c9e2ef5857fef5b3d49454f6240f3
1 file changed