OMAPFB: use dma_alloc_attrs to allocate memory

Use dma_alloc_attrs to allocate memory instead of omap specific vram
allocator. After this we can remove the omap vram allocator.

There are some downsides to this change:

1) dma_alloc_attrs doesn't let us allocate at certain physical address.
However, this should not be a problem as this feature of vram allocator
is only used when reserving the framebuffer that was initialized by the
bootloader, and we don't currently support "passing" a framebuffer from
the bootloader to the kernel anyway.

2) dma_alloc_attrs, as of now, always ioremaps the allocated area, and
we don't need the ioremap when using VRFB. This patch uses
DMA_ATTR_NO_KERNEL_MAPPING for the allocation, but the flag is currently
not operational.

3) OMAPFB_GET_VRAM_INFO ioctl cannot return real values anymore. I
changed the ioctl to return 64M for all the values, which, I hope, the
applications will interpret as "there's enough vram".

4) "vram" kernel parameter to define how much ram to reserve for video
use no longer works. The user needs to enable CMA and use "cma"
parameter.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>