msm: wfd: Stop mapping buffers into kernel memory

Aside for debugging purposes, there's no need to map input/output
buffers into kernel.  Unnecessarily mapping it causes low memory issues
on various targets as the buffers are as big as 2MB each.

As kernel mappings are useful for debugging, map it only when
CONFIG_MSM_WFD_DEBUG has been enabled.

Change-Id: I2c3d48397a0ebdd5c8dc559c306c54bda0b7d076
Signed-off-by: Deva Ramasubramanian <dramasub@codeaurora.org>