drm/amd/display: Remove dml_common_def file

During the rework for removing the FPU issues, I found the following
warning:

 [..] dml_common_defs.o: warning: objtool: dml_round()+0x9: FPU
      instruction outside of kernel_fpu_{begin,end}()

This file has a single function that does not need to be in a specific
file. This commit drop dml_common_defs file, and move dml_round function
to dml_inline_defs.

CC: Christian König <christian.koenig@amd.com>
CC: Alexander Deucher <Alexander.Deucher@amd.com>
CC: Peter Zijlstra <peterz@infradead.org>
CC: Tony Cheng <tony.cheng@amd.com>
CC: Harry Wentland <hwentlan@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>