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

 drivers/gpu/drm/amd/amdgpu/../display/dc/dc_helper.c:299:10: warning: no previous prototype for ‘generic_reg_get’ [-Wmissing-prototypes]
 drivers/gpu/drm/amd/amdgpu/../display/dc/dc_helper.c:307:10: warning: no previous prototype for ‘generic_reg_get2’ [-Wmissing-prototypes]
 drivers/gpu/drm/amd/amdgpu/../display/dc/dc_helper.c:317:10: warning: no previous prototype for ‘generic_reg_get3’ [-Wmissing-prototypes]
 drivers/gpu/drm/amd/amdgpu/../display/dc/dc_helper.c:329:10: warning: no previous prototype for ‘generic_reg_get4’ [-Wmissing-prototypes]
 drivers/gpu/drm/amd/amdgpu/../display/dc/dc_helper.c:343:10: warning: no previous prototype for ‘generic_reg_get5’ [-Wmissing-prototypes]
 drivers/gpu/drm/amd/amdgpu/../display/dc/dc_helper.c:359:10: warning: no previous prototype for ‘generic_reg_get6’ [-Wmissing-prototypes]
 drivers/gpu/drm/amd/amdgpu/../display/dc/dc_helper.c:377:10: warning: no previous prototype for ‘generic_reg_get7’ [-Wmissing-prototypes]
 drivers/gpu/drm/amd/amdgpu/../display/dc/dc_helper.c:397:10: warning: no previous prototype for ‘generic_reg_get8’ [-Wmissing-prototypes]
 drivers/gpu/drm/amd/amdgpu/../display/dc/dc_helper.c:503:6: warning: no previous prototype for ‘generic_write_indirect_reg’ [-Wmissing-prototypes]
 drivers/gpu/drm/amd/amdgpu/../display/dc/dc_helper.c:511:10: warning: no previous prototype for ‘generic_read_indirect_reg’ [-Wmissing-prototypes]
 drivers/gpu/drm/amd/amdgpu/../display/dc/dc_helper.c:529:10: warning: no previous prototype for ‘generic_indirect_reg_get’ [-Wmissing-prototypes]
 drivers/gpu/drm/amd/amdgpu/../display/dc/dc_helper.c:560:10: warning: no previous prototype for ‘generic_indirect_reg_update_ex’ [-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: Noah Abradjian <noah.abradjian@amd.com>
Cc: Rodrigo Siqueira <Rodrigo.Siqueira@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>