drm: tegra: Use framebuffer pitch as line stride

Instead of using the stride derived from the display mode, use the pitch
associated with the currently active framebuffer. This fixes a bug where
the LCD display content would be skewed when enabling HDMI with a video
mode different from that of the LCD.

Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
Tested-by: Terje Bergstrom <tbergstrom@nvidia.com>
Tested-by: Mark Zhang <markz@nvidia.com>
Tested-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>