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.