drm/exynos: Compute dumb-buffer sizes with drm_mode_size_dumb()

drm/exynos: Compute dumb-buffer sizes with drm_mode_size_dumb()

Call drm_mode_size_dumb() to compute dumb-buffer scanline pitch and
buffer size. No alignment required.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Inki Dae <inki.dae@samsung.com>
Cc: Seung-Woo Kim <sw0312.kim@samsung.com>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Cc: Krzysztof Kozlowski <krzk@kernel.org>
Cc: Alim Akhtar <alim.akhtar@samsung.com>
Acked-by: Inki Dae <inki.dae@samsung.com>
Link: https://lore.kernel.org/r/20250821081918.79786-8-tzimmermann@suse.de