drm/stm: fix CONFIG_FB dependency

drm/stm: fix CONFIG_FB dependency

The DRM_STM driver can be built independently of the framebuffer
layer, but it causes a Kconfig warning:

WARNING: unmet direct dependencies detected for FB_PROVIDE_GET_FB_UNMAPPED_AREA
  Depends on [n]: HAS_IOMEM [=y] && FB [=n]
  Selected by [y]:
  - DRM_STM [=y] && HAS_IOMEM [=y] && DRM [=y] && (ARCH_STM32 [=n] || ARCH_MULTIPLATFORM [=y])

Selecting FB_PROVIDE_GET_FB_UNMAPPED_AREA actually has no effect
if CONFIG_FB is disabled, so we can make it a conditional 'select'
instead.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190307155323.2949975-1-arnd@arndb.de