drm/amdgpu/gfx_v8_0: Mark expected switch fall-through

In preparation to enabling -Wimplicit-fallthrough, mark switch
cases where we are expecting to fall through.

This patch fixes the following warning:

drivers/gpu/drm/amd/amdgpu/gfx_v8_0.c: In function ‘gfx_v8_0_tiling_mode_table_init’:
./include/linux/device.h:1487:2: warning: this statement may fall through [-Wimplicit-fallthrough=]
  _dev_warn(dev, dev_fmt(fmt), ##__VA_ARGS__)
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/gpu/drm/amd/amdgpu/gfx_v8_0.c:3236:3: note: in expansion of macro ‘dev_warn’
   dev_warn(adev->dev,
   ^~~~~~~~
drivers/gpu/drm/amd/amdgpu/gfx_v8_0.c:3240:2: note: here
  case CHIP_CARRIZO:
  ^~~~

Warning level 3 was used: -Wimplicit-fallthrough=3

This patch is part of the ongoing efforts to enable
-Wimplicit-fallthrough.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>