gpu: host1x: Fix compiler errors by converting to dma_addr_t

The compiler is complaining with the following errors:

drivers/gpu/host1x/cdma.c:94:48: error:
	passing argument 3 of ‘dma_alloc_wc’ from incompatible pointer type
	[-Werror=incompatible-pointer-types]

drivers/gpu/host1x/cdma.c:113:48: error:
	passing argument 3 of ‘dma_alloc_wc’ from incompatible pointer type
	[-Werror=incompatible-pointer-types]

The expected pointer type of the third argument to dma_alloc_wc() is
dma_addr_t but phys_addr_t is passed.

Change the phys member of struct push_buffer to be dma_addr_t so that we
pass the correct type to dma_alloc_wc().
Also check pb->mapped for non-NULL in the destroy function as that is the
right way of checking if dma_alloc_wc() was successful.

Signed-off-by: Emil Goode <emil.fsw@goode.io>
Signed-off-by: Thierry Reding <treding@nvidia.com>