drm/fb_cma_helper: remove duplicate const from drm_fb_cma_alloc

Duplicated const, only one is required.  Also reformat line to ensure
it is less than 80 columns wide.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1453287574-3645-1-git-send-email-colin.king@canonical.com


Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>