drm: exynos: Remove unreachable code

drm: exynos: Remove unreachable code

This patch reworks function arguments testing to avoid unreachable code
as pointed out with an SVACE warning:

* UNREACHABLE_CODE: This statement in the source code might be unreachable
  during program execution.
   [unreachable] unreachable at drivers/gpu/drm/exynos/exynos_drm_ipp.c:1442
   [The condition property == 0 is always false because at this program
  point the variable property is always not equal to 0] The condition
  property == 0 is always false because at this program point the variable
  property is always not equal to 0 at drivers/gpu/drm/exynos/exynos_drm_ipp.c:1441

* UNREACHABLE_CODE: This statement in the source code might be unreachable
  during program execution.
    [unreachable] unreachable at drivers/gpu/drm/exynos/exynos_drm_ipp.c:1569
    [The condition event_work == 0 is always false because at this program point the
  variable event_work is always not equal to 0] The condition event_work == 0
  is always false because at this program point the variable event_work is
  always not equal to 0 at drivers/gpu/drm/exynos/exynos_drm_ipp.c:1568

Change-Id: I2a9426685c87047bf5ec3ee88b6cce4d1d7d6158
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>