drm: meson: crtc: use proper macros instead of magic constants

This patch add new macros which describe couple bits field of the
following registers:
- VD1_BLEND_SRC_CTRL
- VPP_SC_MISC

Signed-off-by: Julien Masson <jmasson@baylibre.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/86wohb82fa.fsf@baylibre.com