drm/ast: Use new drm_fb_helper functions

Use the newly created wrapper drm_fb_helper functions instead of calling
core fbdev functions directly. They also simplify the fb_info creation.

Cleaned up the error handling in astfb_create a bit.

v2:
- removed unused variable 'device' in astfb_create

Cc: David Airlie <airlied@linux.ie>
Cc: "Y.C. Chen" <yc_chen@aspeedtech.com>
Cc: Alex Deucher <alexander.deucher@amd.com>

Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>