drm/msm/dpu: use full scale alpha in _dpu_crtc_setup_blend_cfg()

drm/msm/dpu: use full scale alpha in _dpu_crtc_setup_blend_cfg()

Both _dpu_crtc_setup_blend_cfg() and setup_blend_config_alpha()
callbacks embed knowledge about platform's alpha range (8-bit or
10-bit). Make _dpu_crtc_setup_blend_cfg() use full 16-bit values for
alpha and reduce alpha only in DPU-specific callbacks.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Patchwork: https://patchwork.freedesktop.org/patch/697898/
Link: https://lore.kernel.org/r/20260112-dpu-rework-alpha-v2-2-d168785911d5@oss.qualcomm.com