drm/i915: Mark expected switch fall-throughs

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

This patch fixes the following warnings:

drivers/gpu/drm/i915/gem/i915_gem_mman.c: In function ‘i915_gem_fault’:
drivers/gpu/drm/i915/gem/i915_gem_mman.c:342:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
   if (!i915_terminally_wedged(i915))
      ^
drivers/gpu/drm/i915/gem/i915_gem_mman.c:345:2: note: here
  case -EAGAIN:
  ^~~~

drivers/gpu/drm/i915/gem/i915_gem_pages.c: In function ‘i915_gem_object_map’:
./include/linux/compiler.h:78:22: warning: this statement may fall through [-Wimplicit-fallthrough=]
 # define unlikely(x) __builtin_expect(!!(x), 0)
                      ^~~~~~~~~~~~~~~~~~~~~~~~~~
./include/asm-generic/bug.h:136:2: note: in expansion of macro ‘unlikely’
  unlikely(__ret_warn_on);     \
  ^~~~~~~~
drivers/gpu/drm/i915/i915_utils.h:49:25: note: in expansion of macro ‘WARN’
 #define MISSING_CASE(x) WARN(1, "Missing case (%s == %ld)\n", \
                         ^~~~
drivers/gpu/drm/i915/gem/i915_gem_pages.c:270:3: note: in expansion of macro ‘MISSING_CASE’
   MISSING_CASE(type);
   ^~~~~~~~~~~~
drivers/gpu/drm/i915/gem/i915_gem_pages.c:272:2: note: here
  case I915_MAP_WB:
  ^~~~

drivers/gpu/drm/i915/i915_gpu_error.c: In function ‘error_record_engine_registers’:
./include/linux/compiler.h:78:22: warning: this statement may fall through [-Wimplicit-fallthrough=]
 # define unlikely(x) __builtin_expect(!!(x), 0)
                      ^~~~~~~~~~~~~~~~~~~~~~~~~~
./include/asm-generic/bug.h:136:2: note: in expansion of macro ‘unlikely’
  unlikely(__ret_warn_on);     \
  ^~~~~~~~
drivers/gpu/drm/i915/i915_utils.h:49:25: note: in expansion of macro ‘WARN’
 #define MISSING_CASE(x) WARN(1, "Missing case (%s == %ld)\n", \
                         ^~~~
drivers/gpu/drm/i915/i915_gpu_error.c:1196:5: note: in expansion of macro ‘MISSING_CASE’
     MISSING_CASE(engine->id);
     ^~~~~~~~~~~~
drivers/gpu/drm/i915/i915_gpu_error.c:1197:4: note: here
    case RCS0:
    ^~~~

drivers/gpu/drm/i915/display/intel_dp.c: In function ‘intel_dp_get_fia_supported_lane_count’:
./include/linux/compiler.h:78:22: warning: this statement may fall through [-Wimplicit-fallthrough=]
 # define unlikely(x) __builtin_expect(!!(x), 0)
                      ^~~~~~~~~~~~~~~~~~~~~~~~~~
./include/asm-generic/bug.h:136:2: note: in expansion of macro ‘unlikely’
  unlikely(__ret_warn_on);     \
  ^~~~~~~~
drivers/gpu/drm/i915/i915_utils.h:49:25: note: in expansion of macro ‘WARN’
 #define MISSING_CASE(x) WARN(1, "Missing case (%s == %ld)\n", \
                         ^~~~
drivers/gpu/drm/i915/display/intel_dp.c:233:3: note: in expansion of macro ‘MISSING_CASE’
   MISSING_CASE(lane_info);
   ^~~~~~~~~~~~
drivers/gpu/drm/i915/display/intel_dp.c:234:2: note: here
  case 1:
  ^~~~

drivers/gpu/drm/i915/display/intel_display.c: In function ‘check_digital_port_conflicts’:
  CC [M]  drivers/gpu/drm/nouveau/nvkm/engine/disp/cursgv100.o
drivers/gpu/drm/i915/display/intel_display.c:12043:7: warning: this statement may fall through [-Wimplicit-fallthrough=]
    if (WARN_ON(!HAS_DDI(to_i915(dev))))
       ^
drivers/gpu/drm/i915/display/intel_display.c:12046:3: note: here
   case INTEL_OUTPUT_DP:
   ^~~~

Also, notice that the Makefile is modified to stop ignoring
fall-through warnings. The -Wimplicit-fallthrough option
will be enabled globally in v5.3.

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

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

Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>