CHROMIUM: drm/exynos: fbdev: Set visible dimensions correctly

CHROMIUM: drm/exynos: fbdev: Set visible dimensions correctly

When probing the fbdev, the drm_fb_helper may specify a visible area
(fb_width x fb_height) than is smaller than the total requested
framebuffer area (surface_width x surface_height).

Use these fb dimensions when setting the visible area in the "variable"
portion of the fb info.  This is consistent with other drm fbdev drivers
(i915, radeon, nouveau).

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
BUG=none
TEST=splash screen displayed properly at boot w/ no HDMI
TEST=splash screen displayed properly at boot w/ HDMI

Change-Id: Id209ea395a7e5eb993632ae4cfe4263b5af18138
Reviewed-on: https://gerrit.chromium.org/gerrit/40443
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
Commit-Queue: Daniel Kurtz <djkurtz@chromium.org>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Tested-by: Daniel Kurtz <djkurtz@chromium.org>