drm/vkms: Fix all kernel-doc warnings of the vkms_composer file

Fix the following warnings:

drivers/gpu/drm/vkms/vkms_composer.c:42: warning: Function parameter or member 'frame_info' not described in 'pre_mul_alpha_blend'
drivers/gpu/drm/vkms/vkms_composer.c:42: warning: Excess function parameter 'src_frame_info' description in 'pre_mul_alpha_blend'
drivers/gpu/drm/vkms/vkms_composer.c:93: warning: Cannot understand  * @wb_frame_info: The writeback frame buffer metadata
 on line 93 - I thought it was a doc line

by correcting variable names and adding function name.

Signed-off-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Maíra Canal <mairacanal@riseup.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20230508220030.434118-2-mcanal@igalia.com