drm/amd/display/dc/dce112/dce112_resource: Make local functions and ones called by reference static

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/amd/amdgpu/../display/dc/dce112/dce112_resource.c:620:22: warning: no previous prototype for ‘dce112_link_encoder_create’ [-Wmissing-prototypes]
 drivers/gpu/drm/amd/amdgpu/../display/dc/dce112/dce112_resource.c:674:32: warning: no previous prototype for ‘dce112_opp_create’ [-Wmissing-prototypes]
 drivers/gpu/drm/amd/amdgpu/../display/dc/dce112/dce112_resource.c:689:17: warning: no previous prototype for ‘dce112_aux_engine_create’ [-Wmissing-prototypes]
 drivers/gpu/drm/amd/amdgpu/../display/dc/dce112/dce112_resource.c:727:20: warning: no previous prototype for ‘dce112_i2c_hw_create’ [-Wmissing-prototypes]
 drivers/gpu/drm/amd/amdgpu/../display/dc/dce112/dce112_resource.c:742:22: warning: no previous prototype for ‘dce112_clock_source_create’ [-Wmissing-prototypes]
 drivers/gpu/drm/amd/amdgpu/../display/dc/dce112/dce112_resource.c:766:6: warning: no previous prototype for ‘dce112_clock_source_destroy’ [-Wmissing-prototypes]
 drivers/gpu/drm/amd/amdgpu/../display/dc/dce112/dce112_resource.c:1027:16: warning: no previous prototype for ‘dce112_validate_global’ [-Wmissing-prototypes]
 drivers/gpu/drm/amd/amdgpu/../display/dc/dce112/dce112_resource.c:1205:29: warning: no previous prototype for ‘dce112_resource_cap’ [-Wmissing-prototypes]

Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Leo Li <sunpeng.li@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Anthony Koo <Anthony.Koo@amd.com>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>