drm/exynos: consistent use of cpp

A recent commit (272725c7) has removed
the use of 'bits_per_pixel' in DRM. However the corresponding Exynos
driver code still uses the ambiguous 'bpp', even though it is now
initialized from fb->cpp[0].

Consistenly use 'cpp' in FIMD, DECON7 and DECON5433 drivers.

Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>