drm/amd/amdgpu: Simplify bitfield operations in gfx v8

This patch introduces a new macro WREG32_FIELD which is used
to write to a register with a new value in a field.  It's designed
to replace the pattern:

tmp = RREG32(mmFoo);
tmp &= ~REG__FIELD_MASK;
tmp |= new_value << REG__FIELD__SHIFT;
WREG32(mmFoo, tmp)

with:

WREG32_FIELD(Foo, FIELD, new_value);

Unlike WREG32_P() it understands offsets/masks and doesn't
require the caller to shift the value (or mask properly).

It's applied where suitable in the gfx_v8_0.c driver to start
with.
Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu.h

@@ -2210,6 +2210,9 @@ void amdgpu_mm_wdoorbell(struct amdgpu_device *adev, u32 index, u32 v);
 #define REG_GET_FIELD(value, reg, field)				\
 	(((value) & REG_FIELD_MASK(reg, field)) >> REG_FIELD_SHIFT(reg, field))
 
+#define WREG32_FIELD(reg, field, val)	\
+	WREG32(mm##reg, (RREG32(mm##reg) & ~REG_FIELD_MASK(reg, field)) | (val) << REG_FIELD_SHIFT(reg, field))
+
 /*
  * BIOS helpers.
  */