staging: sm7xxfb: delete unnecessary color map allocation

This patchs erases fb_alloc_cmap call in smtc_free_fb_info. In the
future, when needed, the required function to call here should be
fb_dealloc_cmap.

Tested with SM712.

Signed-off-by: Javier M. Mellid <jmunhoz@igalia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>