accel/habanalabs: tiny refactor of hl_map_dmabuf()

alloc_sgt_from_device_pages() includes relatively many parameters, and
in a subsequent change another offset parameter is going to be added.
Using structure fields directly when calling this function, and in
hl_map_dmabuf() it is done twice, makes it a little bit difficult to
understand the meaning of the parameters.
To make it clearer, assign the required values into local variables with
explicit names, and use the variables when calling the function.

Signed-off-by: Tomer Tayar <ttayar@habana.ai>
Reviewed-by: Oded Gabbay <ogabbay@kernel.org>
Signed-off-by: Oded Gabbay <ogabbay@kernel.org>