tinydrm: repaper: add CONFIG_THERMAL dependency

The new RePaper driver uses the thermal subsystem, and fails to link
when it is built-in but thermal is a loadable module:

drivers/gpu/drm/tinydrm/repaper.o: In function `repaper_probe':
repaper.c:(.text+0x540): undefined reference to `thermal_zone_get_zone_by_name'
drivers/gpu/drm/tinydrm/repaper.o: In function `repaper_fb_dirty':
repaper.c:(.text+0xff4): undefined reference to `thermal_zone_get_temp'

This adds another Kconfig dependency to prevent the broken configuration,
forcing repaper to be a module too.

Fixes: 3589211e ("drm/tinydrm: Add RePaper e-ink driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20170727100004.300665-1-arnd@arndb.de