gralloc: Allow system heap to be uncached.

Remove an old check that prevented this from happening.

Change-Id: I938b11b27e58fc38a679dc55e31e7b51eb86bd3a
libgralloc: Allow USAGE_RARELY flags to be uncached

Allow GRALLOC_USAGE_SW_READ_RARELY and GRALLOC_USAGE_SW_WRITE_RARELY
flags to be uncached.

Change-Id: I014d659e5d5465c6a27aa3979d52cbf3f7c14024