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>