drm/i915: Remove set but not used variable 'src_y'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/gpu/drm/i915/display/intel_sprite.c: In function 'g4x_sprite_check_scaling':
drivers/gpu/drm/i915/display/intel_sprite.c:1494:13: warning:
 variable 'src_y' set but not used [-Wunused-but-set-variable]

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190719024100.64738-1-yuehaibing@huawei.com