vc4/drm: Fix source offsets with DRM_FORMAT_P030

Spec says: bits [31:4] of the given address should point to
the 128-bit word containing the desired starting pixel,
and bits[3:0] should be between 0 and 11, indicating which
of the 12-pixels in that 128-bit word is the first pixel to be used

Signed-off-by: Dom Cobley <popcornmix@gmail.com>