drm/omap: remove set but not used variable 'frame_height'
Fixes gcc '-Wunused-but-set-variable' warning:
drivers/gpu/drm/omapdrm/dss/dispc.c: In function 'dispc_ovl_setup_common':
drivers/gpu/drm/omapdrm/dss/dispc.c:2627:19: warning:
variable 'frame_height' set but not used [-Wunused-but-set-variable]