drm/amd/display: correct alpha_en programming for new pixel format

[why]
for following new format, no alpha
       SURFACE_PIXEL_FORMAT_GRPH_RGB111110_FLOAT/_FIX:
       SURFACE_PIXEL_FORMAT_GRPH_BGR101111_FLOAT/_FIX
       same as case SURFACE_PIXEL_FORMAT_GRPH_ARGB1555:

Signed-off-by: Charlene Liu <Charlene.Liu@amd.com>
Reviewed-by: Chris Park <Chris.Park@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>