If CMA is turned on and CMA size is set to zero, kernel should
behave as if CMA was not enabled at compile time.
Every dma allocation should check existence of cma area
before requesting memory.

Arm has done this by commit e464ef16 ("arm: dma-mapping: add
checking cma area initialized"), also do this for arm64.
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Jisheng Zhang <jszhang@marvell.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>