OMAPFB: remove warning when trying to alloc at certain paddress

omapfb gives a WARN_ONCE if a predefined physical address is given for
allocating the framebuffer memory, as this is not currently supported.

However, the same warning happens if omapfb fails to allocate memory
during runtime, as when the allocation has failed, omapfb tries to
re-allocate the old memory with the physical address of the old memory
area.

Remove the warning from omapfb_alloc_fbmem, as it serves no purpose on
the failure case above, and move it to omapfb_parse_vram_param, so that
we only warn if physical address is given via omapfb module parameters.

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