core: Move DmaBufAttributes getter to GraphicsBuffer
LinuxDmaBufV1ClientBuffer contains properties (formats, and flags) that are not available in the base GraphicsBuffer type and there's no reason to move it there.
In order to get rid of those properties (and eventually hide the LinuxDmaBufV1ClientBuffer type from the public api), this change adds a DmaBufAttributes getter in the GraphicsBuffer.