mm: cma: Always consider a 0 base address reservation as dynamic

The fixed parameter to cma_declare_contiguous() tells the function
whether the given base address must be honoured or should be considered
as a hint only. The API considers a zero base address as meaning any
base address, which must never be considered as a fixed value.

Part of the implementation correctly checks both fixed and base != 0,
but two locations check the fixed value only. Set fixed to false when
base is 0 to fix that and simplify the code.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>