video: pxafb: Fix the function used to balance a 'dma_alloc_coherent()' call

'dma_alloc_coherent()' must be balanced by a call to 'dma_free_coherent()'
not 'dma_free_wc()'.
The correct dma_free_ function is already used in the error handling path
of the probe function.

Fixes: 77e19675

 ("[ARM] pxafb: allow video memory size to be configurable")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Viresh Kumar <viresh.kumar@linaro.org>
Cc: Jani Nikula <jani.nikula@intel.com>
cc: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Cc: Eric Miao <eric.miao@marvell.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200429084505.108897-1-christophe.jaillet@wanadoo.fr