drm/meson: overlay: fix build failure

drm/meson: overlay: fix build failure

The recent GCC compiler is very picky with the VD_H_START() and
AFBC_DEC_PIXEL_BGN_H() macros, triggering a runtime assert error as:

In function 'meson_overlay_setup_scaler_params',
    inlined from 'meson_overlay_atomic_update' at
drivers/gpu/drm/meson/meson_overlay.c:542:2:
./include/linux/compiler.h:392:38: error: call to
'__compiletime_assert_341' declared with attribute error: FIELD_PREP:
value too large for the field

drivers/gpu/drm/meson/meson_overlay.c:413:4: note: in expansion of macro
'AFBC_DEC_PIXEL_BGN_H'
  413 |    AFBC_DEC_PIXEL_BGN_H(hd_start_lines - afbc_left) |
      |    ^~~~~~~~~~~~~~~~~~~~
./include/linux/compiler.h:392:38: error: call to
'__compiletime_assert_401' declared with attribute error: FIELD_PREP:
value too large for the field

It's not expected to overflow these fields, but the compiler did
find a case where it overflows.
We can safely ignore this, so mask the value with the field width.

Fixes: e860785d57306 ("drm/meson: overlay: setup overlay for Amlogic FBC")
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
[narmstrong: moved to (value) to avoid precedence issues]
Link: https://patchwork.freedesktop.org/patch/msgid/20200707135009.32474-1-narmstrong@baylibre.com