drm/vkms: fix xrgb on compute crc

The previous memset operation was not correctly zeroing the alpha
channel to compute the crc, and as a result, the IGT subtest
kms_cursor_crc/pipe-A-cursor-alpha-transparent fails.

Fixes: db7f419c ("drm/vkms: Compute CRC with Cursor Plane")

Signed-off-by: Melissa Wen <melissa.srw@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200730202524.5upzuh4irboru7my@smtp.gmail.com